Unity was never open source, right? Different situation.

More like mongodb or elasticsearch

Not nearly as extreme. I can understand the comparison but it's not even close to the egregiousness (that's a word, right?) of unity

They can't retroactively change the license, doubly so if it's an open-source license, as that would break the contractual agreement made with existing users when they accepted the license terms.

And if the previous license used allows for redistribution, then the existing users are still allowed to redistribute the software under the previous terms, which makes the previous license still valid for new users since they don't have to acquire the software from the original source and are bound to the license it originally came with.

It's a legal can of worms you don't want to open.