Yes, BMS systems aren’t in every tank, they’re usually in a couple of tanks in a certain outfit (CO, XO etc.).
There’s the hardware for the BMS and the software for it. In older tanks such as the AK you need to add a special computer to run the BMS on, it’s basically a rugged laptop/tablet. In newer tanks the Commanders standard displays and computers will have this functionality built in (VT-4).
If a new BMS is introduced, as long as the older hardware is capable of running it, they can just run the new software on the older machines, or even make the older software compatible with the newer one. The programmers that work on these shouldn’t have an issue doing that because they’re on the same network.
The real problem with our older BMS systems wasn’t really capability, it was ergonomics, most mainstream military tech is a headache to use, this isn’t limited to Pakistan, even American and German tankers have admitted to not using many of the digitization features because they’re slow and annoying. I hope they new hardware and software is intuitive and fast enough to where commanders actually feel encouraged to use it.