Then, how likely is it that the format of transactions, generated by the watching version to be signed, will change?
There's no absolute answer to this. Highly unlikely I suppose, why would the format ever need to be changed? You shouldn't worry about the format of the transactions, but the new features that are added on top of the Bitcoin protocol overtime, and that you can't use in 3.3.8, such as Taproot.
Or, is it 'set in stone' and fixed by the Bitcoin protocols?
I don't think it'll ever will, and even if it does, the network will treat the old format as non-standard. Currently, there are thousands of Lightning channels opened and people who've signed transactions for inheritance. Suddenly rejecting the old format would destroy these applications.