Running Wire to an Outside Load Center
Yes, you can change the panel and the circuits will remain grandfathered.  Since you are not working on the circuits themselves. 

It would need to be that way, otherwise replacing a dangerous FPE or Zinsco panel would leap from a $300 job to a $1500 job.  But just to be sure on that, I would check with your AHJ. 

Regardless of that, don't plan on using double-stuff breakers. Size the panel so every now and future circuit has a whole space, or 2 spaces for 240V.  Also give yourself some spaces for unanticipated future expansion. Plus 4 more for a gen interlock and a whole house surge suppressor.