Quote from: ps1234 on Today at 03:56:24 PM

Then, how likely is it that the format of transactions, generated by the watching version to be signed, will change?

I don't think anyone can answer this. Highly unlikely I guess so, 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, such as Taproot.

Quote from: ps1234 on Today at 03:56:24 PM

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 future usage. Suddenly rejecting the old format would destroy these applications.