Simple Mobile apps have been very popular among FOSS enthusiasts. I've personally been using the Gallery, Contacts and the Phone app since a few years now. It's a shame that it has come to this, will be on the lookout for their forks.
From the comment thread it sounds like the contract might be on shaky ground. The original dev seems to be under the impression that a project licensed under GPLv3 can just be freely changed to another license, which it cannot without explicit consent from all contributors. We don't know the contract language of course, but the dev said it would "likely" not remain open source, which indicates that he told them they could change it. ZippApps' lawyers will hopefully notice that and refuse to buy, though either way we shouldn't trust this maintainer again and should move to the forks.