[Ashur]

most important thing is to backup your database and probably you would do it from PhpMyAdmin

out of all files the most important are the torrents folder everything else has lower importance especially if you didn't make any changes to the code if you did then i guess you need to backup all the files,
If you think it's useless to backup then you're wrong it's always good to backup :)

Here are few questions to help you better!!!
1. are you keeping the same domain name?
2. are the database, username and password going to change?
3. is there a special smtp setting needs to be followed?

once you answer and provide whatever information necessary i or somebody else can you help you better