Reinstalling from Time Machine, what changes?
As a rule of thumb system files are preserved from the fresh install and your data is copied from the Time Machine.

You've mentioned file associations — I presume you're referring to file associations with your own files, and therefore a Time Machine restore would copy those associations. The main exception to this is if the application that the file is associated with is missing. Then the file would change it's association to the 'next best' app.



From the rule of thumb you can generally guess what'll be restored and what'll be left alone, but feel free to add more specific detail to your question.