Currently my laptop is CentOS 6.2, which I'd like to change to Fedora 17. However, rather than simply replacing my current OS, I'd like to dual boot Fedora 17 and run it for a few weeks to ensure I like it and it works for me before I get rid of everything I set up in CentOS (which after countless hours of setup/configuration works well for me). At that point, I will resize the Fedora partitions to be my whole disk.

(By the way, is dual booting, even though it's temporary, a good idea when my old laptop has only an 80GB hard drive? Would I just give Fedora half, so 40GB?)

So I'm backing up my current system before I try anything, but I'd like to know what the best way to get to dual booting is. Should I just pop in the installation DVD for Fedora 17 and let it do all the partitioning, etc, for me? Or is it better for me to do this from CentOS rescue mode and then install Fedora into the newly created space? If the latter, which commands should I look at using for this?

Currently I don't use encrypted partitions, but let's say I did, what would change to the above process? (I'd like to use encrypted partitions when I install Fedora, and am wondering if it will make it difficult when I try to dual boot some other Linux with it in the future).

A truly excellent idea! Since you have spent a lot of time on setup and configuration of preferences, you might also consider making seperate backups of just your users home directory, as well as the root users directory (if you have also made such configs for the root user). Once Fedora is installed, you will probably be using the same apps, so just restore your home and root backups to the Fedora installation to have all your configs/preferences in place.
You could probably get by with a 15 to 20 gig partition for Fedora unless you intend to copy a lot of files from the Centos installation to the Fedora partition. Otherwise, a 40/40 gig split should be more than adequate.
If it were my system, I'd use a liveCD with gparted to do the partitioning before installing Fedora, then just direct Fedora to use the new partition on installing Fedora.
No, no, NO! That would put you in the position of trying to partition a running OS. Definitely not a good idea. Partitioning should be done on an unmounted filesystem.

A final note, when you install Fedora, make certain that when you get to the point you are asked where to install grub, install to the Fedora partition and not to the MBR. That will not add Fedora to the Centos grub menu when you boot. You will have to boot into Centos and run update-grub to add Fedora to the grub menu. Then you can reboot to Fedora to conplete the installtion, if necessary.
If you install Fedoras grub to the MBR, it becomes the controlling installation of grub and will cause you a few problems if you decide you don't want Fedora after all.

Thanks for the responses.

Backing up the home directory, would that work if versions of programs have changed significantly (which they have between CentOS 6.2 and Fedora 17)? Is there risk of things 'breaking' badly, or is there no harm done in the worst case scenario?

Why wouldn't you use Fedora DVD automatic partitioning, just out of curiosity?

Are there any partitioning utilities as part of the rescue mode of Linux DVDs (in this case, Fedora) that I could use so that I don't have to create and keep around another CD (for gparted)? What other partitioning utility options do I have 'straight out of the box'?

I'm a bit confused about this. So in the end, how many partitions would I have? Or I should say LVM partitions...

And if I decide I don't want CentOS after Fedora is running smoothly for a while, would it be a big problem with regards to grub?

Thanks so much.

The files (hidden dot files) in your home directory store the preferences you set when you run an application. These are your personal preferences as apposed to the defaults which are located in /etc. The worst that would likely happen is that there may be new options available in a newer version on an app for which you haven't yet set a preference. To my knowledge, the preference files in your home directory are not version specific. That may not be true for all apps, but it should be true for most apps.

Early in my Linux experience (circa 1999-2000) I made some disastrous mistakes with partitioning that way. So, I learned to protect myself from myself by partitioning via liveCD/liveDVD before installing a distro.
Given the volume of apps in a DVD sized .iso, I'd expect that it probably has gparted, parted, fdisk, cfdisk, and/or sfdisk for partitioning. I prefer the GUI app such as gparted. F/c/s/disk are more DOS type apps that I've used, but, as I say, I prefer gparted. If the DVD runs as a liveDVD that let's you run Fedora without installing, you could use gparted to repartition your disk (remember that the partition you propose to repartition *MUST* be unmounted for partitioning).

With regard to installing grub to the MBR versus installing to the root partition:
The MBR would hold the partition table, and a pointer to the rest of grubs files (especially the config file) which has the menu with pointers to the locations of other operating systems. So, you designate one Linux installation to be the master with a grub config which points to other OSs. Those others have their grub installed to the root partition of the distro. The master then hands off the boot process when you select which one you want to boot into.

If you should delete, by whatever method, the distro which is the master, when you try to boot grub will not have the config files to tell it where the bootable systems are located.

Your choice then is to choose which distro will be the master. Any new distros you install after that will not be listed in the master until you boot into the master and run update-grub (which runs a script called os-prober to locate all bootable distros) and update the grub menu of the master.

Going back to your original post and my first answer, you should end up with three partitions: centos, fedora, and swap (one swap will work for all distros since the only one using swap would be the one you boot into).

No, just boot into Fedora and run update-grub from there to rewrite the grub pointer in the MBR to point to Fedora, then delete/reformat/whatever the Centos partition. You would probably Centos showing in the grub menu from Fedora, but running update-grub again after deleting Centos will correct that.

Thanks for the clarification. I don't have an update-grub command in my CentOS. Is this something that is only available in rescue mode perhaps, or should I install something for this?

If I've understood correctly, update-grub is a script that removes uninstalled partitions from the menu, and makes the default OS (i.e. the first in the grub menu) the OS from which the command is run?

I recall another post indicating that "update-grub" is specific to Ubuntu and derivatives and that "grub-mkconfig" can be used on other distributions with Grub2. Also, yesterday, I saw a post indicating that Fedora used "grub2-mkconfig". You might try those commands. I'm not a Fedora user so have no way to test and the only systems I have which use Grub2 are Ubuntu derivatives.

update-grub is specific to grub2. The older version of grub (grub legacy) doesn't have it.

If you are running grub legacy, you can find the manual, with instructions for configuring /boot/grub/menu.lst, here.