How do I upgrade from 32-bit to 64-bit on Ubuntu?
 

I installed Ubuntu a while ago when I didn't know the differences and because it says 33-bit was recommended. I have 6GBs of RAM and I can't use it all. Do I have to do a fresh reinstall or can I upgrade? If I have to do a fresh reinstall, is it possible to keep all of my settings by backing everything up? How do I find a free online backup program?

If you want 64 bit, you should do a fresh install.

If you want to use all of 6GB, you can do that in 32 bit by installing the PAE kernel (which you can find in the package manager).

With the PAE kernel any single process can only use 3GB, but 6GB can be used by two or more processes. Typical use of Linux would not involve half your memory used by a single process anyway. So the 3GB per process limit is unlikely to be an issue, but that ultimately depends on how you use your computer.

64 bit might be a little faster or a little slower than 32 bit PAE (depending on what applications you run). faster is more likely, but probably not by enough to justify switching. Getting the 32 bit PAE kernel is an easy step from where you are now. Switching to 64 bit is starting over (which you might or might not consider hard at this point).

You can see all 6GB if you install a PAE kernel. PAE does have limitations though, its limited to 3 or 4GB of RAM per process. Its also wont give you the speed advantages of 64bit.

To change from 32bit to 64bit, you would need to reinstall. You cant upgrade from 32bit to 64bit.

Ordinary 32 bit PAE has the same (3GB) limit per process as non PAE 32 bit.

The type of PAE kernel that allows 4GB per process is an obsolete kludge that you wouldn't want even if it were available in Ubuntu (if you can get it at all it would be by custom building your own kernel).

If you need more than 3GB per process, use a 64 bit kernel.

Because "upgrading a system from 32-bit to 64-bit is, or can be, anything but a trivial undertaking," the best thing indeed to do is to start with a new installation.

If you have only one computer to work with, seriously consider buying another hard drive, if you can boot from multiple drives. You would like to be able to build-out the 64 bit configuration without committing yourself irretrievably to it.

I started with a hard drive well over double the size I initially needed. I think most people do (because hard drive pricing tends to suggest doing so). Did you?

When I have needed a fresh reinstall but preserving settings etc. (not for the reason you might, but the reason is irrelevant), I used the GUI partitioner while booted in a liveCD to shrink my partitions to leave enough unpartitioned space for the complete new install.

After doing a new install turning that unpartitioned space into additional partitions (and enabling dual boot with the old install). I mounted the old partition into the new system and copied everything that I knew I wanted to preserve. Then I tried to use it and noted unintended differences. Then I could reboot the old system and figure out what made something work the way I wanted in the old system that I hadn't understood needed copying to the new one (then copy it).

A complete bootable old system to retest after testing the new system is a great resource for anyone who doesn't 100% know what they need to get a migration right.

I much prefer the idea of repartitioning as I described above.

But if your hard drive is so absurdly small that on an apparently fairly new Linux install you don't have room to repartition enough space for a redundant install, then I have roughly the same advice as sundialsvcs: Buy a decent size disk drive. They don't cost very much.

I'm not changing my main advise in this thread: Staying with 32 bit and switching to PAE is much easier. The performance difference is probably trivial. The 3GB per process limit almost certainly won't matter.

But when you do decide to reinstall (now for this reason or whenever for whatever reason) it is best to repartition so you can still retest the old install after initial test of the new install.