I have very little knowledge on this, but I don’t think a hard fork would be possible in the same way as it is with a blockchain.
If anyone decided to run incompatible software on the main Safe network, they won’t be able to access the data, so it won’t be a useful network except for new data & users who are using their software.
How the network will update to new software is an interesting question, and there are some good threads on updates, though as far as I can see there’s no specific outline of how this will work yet:
Having periodic updates will I’m sure be necessary to keep improving the capabilities of the network, and I’ll be very interested to hear how this process will work.