On SE, contribution licensing was a sticking point, especially when the sites were relicensed.
Might I suggest that each site have a list of acceptable content licenses, both current and former. Each post made should be made under one of the then-current licenses (as selected by the user if more than one is current) and that licence referenced in the contribution record.
Then, if the site changes its licenses(s), any pre-existing contribution remains under its original license unless the licensor decides to update its license.
This has the effect that each contribution is explicitly licensed under a known set of license T&Cs, and that relicensing cannot change existing licence conditions.