IANAL. I have no ability to interpret laws. I must defer to the state's expertise.
From the document at the link:
8. Will I need to register all my firearms?
a. Yes. However, you will not be required to register your firearms into the new registration system until 10/2/2026. The Commonwealth is working on building the new registration system and will provide additional information about how to register firearms at a later date. In the meantime, users may register firearms on the existing Massachusetts Gun Transaction Portal at
https://mircs.chs.state.ma.us/fa10/action/home?app_context=home&app_action=presentHome.
If you have already registered your firearms on the current transaction portal, you will not have to re-register them when the new system goes live.
Cool. The state's office enforcing the law has now explicitly said that all your existing home builds that you entered on the current portal within 7 days of their becoming capable of firing do not need to be registered on the new system.
Everyone should save a copy of that document in its current state before they change it..