Upgrade from 6.2 to 6.4 - Possible
A repository migration will be required due to SegmentNodeStore change in 6.3
You can find all the details and steps here:
We did not face any issue while migration as ours is a light weight application in terms of number of dynamic content components that uses integrated services.
The issue that we faced was because of our application content package that had junk/repeat content and junk references. We had to reconstruct our content package but it had nothing to do with 6.4