There is an exception to the ex-post-facto ban for gun laws.
When the Lautenberg amendment (misdemeanor domestic violence conviction => federal lifetime prohibited person), deprivation of 2A rights was not viewed as punishment, and applied retroactively. I know of no other case in which a legally enshrined right was removed because it was added to the penalty for an old conviction prior to the law change.
On the administrative side, MA applied an ex-post-facto standard to drivers license penalty points when they went from a 3 to 6 year lookback - and resurrected tickets people had seen "age off" their driving record.
There are currently three situations in which CWOF may be overtly used against an in-duh-vidual:
- CWOF for OUI is counted as a predicate conviction
- CWOF for driving crimes are considered convictions for commercial drivers license issues. The feds prohibit any alternative adjudication other than not guilty/dismisses charges that avoids the conviction of a sanction), and the MA SJC held this applies to CWOF
- FIrearms licensing suitability
There is nothing to prevent MA from adding other ex-post-facto penalties for pre-existing CWOFs in the future.