In simpler terms:
The requirement of Planning Permission was only brought in in '63. Therefore anything that you built before '63, did not require Planning, and so you don't have to worry about it. If the unit was split into multiple units before '63, then that was ok, if the unit was split into multiple units after '63, then it should have had Planning.
Building Regulations came in much later, initially as guidelines and then compulsorily in '92, I think, as a result of the Stardust tragedy.
I don't understand what you mean when you say "lose pre '63 status". "Pre '63 status" means no problem with the Planning Permission. Unless the renovations in and of themselves require Planning Permission (as material alterations or change of use not subject to an exemption), how would you lose "pre '63 status"?
Edit: oops - crossed.