Is it possible to change passwords temporarily

I don't believe that's possible. You could have two entries in /etc/passwd with the same user names but different UIDs, but the system would probably just ignore the second one (or misbehave in some way); arguably such an /etc/passwd file would be considered corrupt.

When you login to the system, you're first prompted for your user name. Once you've done that, the system prompts for your password, and checks whether the entered password matches the password for the account corresponding to that user name. By the time you're entering your password, the system has already determined what account you're trying to access.

I suppose you could modify various pieces of the system to get the behavior you want, but you'd have to replace several different pieces of software, including anything that authenticates and authorizes users (console login, su, ssh, and whatever other methods are enabled). Any mistakes would likely open huge gaping security holes.

EDIT : Based on the comments, PAM is probably the way to do this. I'm not familiar enough with PAM to go into more detail. (It's still a really bad idea.)

You can't.

Whatever you do, your users can bypass it and store their password in a plain text file anyway. If you disable the feature in the client binary, they'll download or compile a different client. As a rule, if you set up obnoxious security measures (such as having to type a password for every svn operation), your users will bypass them in a way that makes security worse. (For example, writing a wrapper script that contains their password. Which they'll leave world-readable.) So don't do that.

To reiterate: you cannot, by technical measures alone, prevent users from storing their password in a file. You can forbid it, but if it makes their life difficult, they'll do it anyway.

If you're concerned about laptop or backup theft, encrypt the users' home directory. This will protect the passwords as well as the data. If the whole home directory is encrypted, the encryption password is usually the same as the login password, for usability reasons. Be sure to have a password backup policy (e.g. a sealed envelope), since losing an encryption password is irrecoverable.

If you're concerned about password reuse, impose a random (hence unique) password, which they will type once and for all into their client. Have a simple process for changing compromised passwords, of course.