If you are swapping out RAM to maintain your current level of performance, or upgrading the OS, etc... you are maintaining your PC.
If you are swapping out the GPU from a 3090 to a 5090, you are no longer "maintaining" your PC... you are upgrading it. There is a difference.
I don't go into get my car maintained and come out with new cat-back exhaust.
A team is most certainly "allowed" (next thing we'll hear that allowed means something else) to maintain their roster under the CBA. The Celtics absolutely could have retained their entire roster. They chose not to. Whether or not the Celtics could have upgraded their roster is another question. That's the distinction, and it matters because this entire conversation was me pointing out the premise "the CBA does not allow you to maintain your roster" is objectively false. Now, if the premise was "the CBA does not allow you to improve/upgrade your roster, that is another matter... and it's not the one I was addressing (because it's not the one that was presented).
Anyway, I'm going to move on to other conversations in hope the quality of discourse maintains.