Most of it boils down to your database, the way user passwords are hashed, etc. Little has to do with your script files.

For the most part, most companies will either provide you with a migration script (e.g., the forum-software market), or they will offer data-migration services at a fee (from a few bucks to several hundred, depending on the db's complexity). You can always hire a third-party, of course.

Anything can be done, and no, you are not to be forever "married" to a particular see more script; its just hard work and may get expensive.