Can you change the robot after participating in the regional stage to the national stage?
That will be a question to ask to your regional representative to better understand the premises on which they distribute the slots to the international competition and the expectations.
Diego Garza Rodriguez
Contact details for regional representatives are available here, scroll to bottom for regional reps, contact yours to be completely sure.
That being said I can tell you with certainty that for the international tournament as well as my regional tournament (Germany), the European Superregional and a couple of other European regions (which are all the places I personally have experiences with) there are no limits to modifying the robot at any time before, after or even during any particular competition (except that to be allowed to compete is has to conform to the rules of course) and improving your robot all the time until the very end is normal.