Rebuilding system and tables, should I change primary key to int?
No, it is complete overkill for the size of this system and an unnecessary complication for an established database. As for "all the reasons that using an int PK is a good idea", those reasons only apply in either VERY large systems (the performance argument), in systems without CASCADE UPDATE on FKs (the immutable argument), or where a overly-simplistic ORM (or similar style) is in use (the "object identifier" argument).

Keep it simple and work within the existing design.  You're just creating a ton of extra work and risk of errors if you try and retro-fit a new set of keys onto the existing data.