Does deleting lower subdivisions free up RAM so that we can start subdividing a model further without causing ZBrush to crash?
I’m not positive on that but I would say no, If you delete lower subdivisions you do not reset the tools overall poly count basically what happens is you will lock in the poly level you currently have after deleting the lower ones. In other words if your a sub division 3 and at 1 million polys and then delete sub division levels 2 and 1 your poly count will be 1 million and will not be able to be lowered from that point. Adding more subdivisions at this point would raise your poly count even higher. You can raise from there but be aware that the poly count goes up as does the drain on resources on your system. Eventually it could pose crashes it all depends on your system configuration and memory usage. IMO
The most efficient way to work is with multiple subtools. Having one huge subtool with allot of detail on it is more of a ram gobler than having a tool that is split into many smaller subtools which are subdivided as needed for their detail requirements.