I can't see anything ridiculous on change one's mind - not matter how long an addon has been public available or what the previous license was. It's the authors decision. Period.
The VEM author is free to invest his/her own time in further developing the forked version.
Personally I would take a different road. I would add the VEM stuff/features to DBM instead of changing the license.
But again, it's his/her decision.
[e]
I really would like to know what the rake on Curse rewards looks like. I always thought it couldn't be more than a few dollars - even with big download numbers.