They can’t retroactively close-source the older versions released under AGPL, but if it ever required a community fork to continue the last release of the GPL version, it would be a massive burden to maintain it, and could cause federation to break as the codebase diverges over time, which would create a rift in the community. You’d also have to hope that average users care enough about the license to jump ship to the GPL (probably now not as full-featured) version, otherwise the GPL version risks not being able to get enough funding to continue, or enough users to convince the larger communities to move over.
As a somewhat similar real world example, the pixel-art program Aseprite once used a FLOSS license, but it switched to a proprietary license at some point. The last GPL version was forked by the community, but it never got much traction, and is now massively behind the closed source version in features and userbase.
They can’t retroactively close-source the older versions released under AGPL, but if it ever required a community fork to continue the last release of the GPL version, it would be a massive burden to maintain it, and could cause federation to break as the codebase diverges over time, which would create a rift in the community. You’d also have to hope that average users care enough about the license to jump ship to the GPL (probably now not as full-featured) version, otherwise the GPL version risks not being able to get enough funding to continue, or enough users to convince the larger communities to move over.
As a somewhat similar real world example, the pixel-art program Aseprite once used a FLOSS license, but it switched to a proprietary license at some point. The last GPL version was forked by the community, but it never got much traction, and is now massively behind the closed source version in features and userbase.