77Punker,
Yes, apt-get dist-upgrade or just upgrade will replace 3.1.5 with 3.2 (assuming, however, that 3.2 is in a repository that you list in your /etc/source.list file). Right now, if you have an experimental repository listed (which I think every mirror has, they just don't list experimental officially on debian.org). In probably a week or two, depending on bugs found, 3.2 will be in unstable, in which case you (obviously) need an unstable repository listed.
By the way, seeing that you're, I gather, new to Debian, I'd suggest that if you have broadband, just put all the repositories (stable, testing, unstable, and experimental) in your sources.list file. Then, in /etc/apt/apt.conf, put this line in (replace 'testing' with whatever you want your default to be): ' APT:
efault-Release "testing"; '. From then on, apt-get will assume your default, but you can also use the useful command 'apt-get -t experimental install kde' to, in this example, install KDE from experimental (so you'd get 3.2).