As reported in a reddit post and confirmed in a github discussion, Simple Mobile Tools is being sold to ZippoApps which is known for shady business practices.
It is not yet clear whether they will make the app suite closed source (which would infringe the rights of all contributors since they contributed under the GPLv3 license).
In response to that situation, one of the main contributors forked the project under the name FossifyX and will continue to work on it.
This is open source at its best: the original developer somehow decided to sell out to the dark side and someone rescued the projects within a couple of days. Brillant!
And thank goodness for that too: I used SMT Calendar but Etar misses a couple of features I really need that would make it a good replacement, so I’ll be sure to install FossifyX Calendar as soon as F-Droid picks it up.
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(IANAL)
Since the software is already distributed under the terms of the GPLv3 (which guarantees irrevocable rights) there is no way to forbid any distribution of the current version of the software.
It is however possible to distribute future works under a different license, but only if you aren’t bound by the GPL yourself. This would be the case if you wrote the code yourself or all contributors grant you the right to do so (eg. using a Contributor License Agreement).
There are clever ways to split software via different abstractions that allow to avoid some license features from affecting future developments too. That is to add new closed source features without relicensing the existing codebase.
Once someone exists publicly as code with an attached valid license it cannot be retroactively removed the right to use it. So only new versions could have different licenses or something.
Some folks seem to forget this feature of open source when it comes to projects like Chromium. History has plenty of examples of forks of small and large projects. Chromium (Blink) itself is one.