The Stack Overflow case [1] that Thomas linked to in another comment seems a good place to learn from.
I think multiple license support on a post-by-post basis is a must. Old posts must be licensed as all-rights-reserved, except for the right of publication on the Forum (which is understood that the authors have granted de facto when they published).
New posts can be required to use a particular license or (even better) users can choose what license to use, with the default being preferably CC-BY per the discussion on other comments.
The license on all posts should be ideally updatable at will, and I would see it as positive to nudge users to update the license in old posts to CC-BY (perhaps sending them an email or a popup next time they log in that gathers their explicit permission to do so).
The Stack Overflow case [1] that Thomas linked to in another comment seems a good place to learn from.
I think multiple license support on a post-by-post basis is a must. Old posts must be licensed as all-rights-reserved, except for the right of publication on the Forum (which is understood that the authors have granted de facto when they published).
New posts can be required to use a particular license or (even better) users can choose what license to use, with the default being preferably CC-BY per the discussion on other comments.
The license on all posts should be ideally updatable at will, and I would see it as positive to nudge users to update the license in old posts to CC-BY (perhaps sending them an email or a popup next time they log in that gathers their explicit permission to do so).
[1] https://meta.stackexchange.com/questions/333089/stack-exchange-and-stack-overflow-have-moved-to-cc-by-sa-4-0