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.
(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.