I raised precisely this concern a week ago: https://news.ycombinator.com/item?id=25631073

my intention there was (and still is) to learn how other HNers think of this. In there I got the response about how the precise version to which people contributed, was and will always be Open Source, regardless of what happens to future derivations of that code.

I'm not sure I bought that reasoning, though... you put it better with that "fruits of their labor". Maybe not the writing, but the spirit of the permissive FOSS licenses is not to end up being swapped into a non-FOSS alternative...

The previous FOSS license did indeed permit any future change in licensing; I would like to learn if that kind of choice might actually become a deterrent and an added reason for a lower number of external contributors, or not.