In our experience, all of the contributors were happy to give permission to re-license the code. They generally believed in the project (that's why they contributed) and wanted TerminusDB to be as successful as possible - if the core team felt that was going to happen by moving to Apache, they weren't going to stand in the way.
I suppose you could have somebody quite ideologically attached to the GNU/GPL who could then hold up a change. If the code was core and you couldn't easily re-implement, could make the price high.
We had the same experience with the Linux kernel. The original license was even more hard-line that the GPL --- it ruled out any way of making money, even selling CD-ROMs with Linux on it. Linus had to consult with all of the contributors to relax the restrictions to GPLv2, and we all agreed that it was in the best interests of the community. So there was no problems in terms of getting consent from all of the developers to make that license change to something more permissive than the original license.
At the time, the failure modes of the more permissive licensing model (for example, of NetApp's innovations for things like WAFL were never contributed back to BSD) were more foremost in our mind, and so for us the GPLv2 was the right compromise between the rights of the community and the companies using, and improving, the code. Of course, the decision that we made in 1992 was in a world very different from 2020, and so hopefully the choice of the Apache license will be the best one for TerminusDB. I have only the best wishes for you and your community!
I suppose you could have somebody quite ideologically attached to the GNU/GPL who could then hold up a change. If the code was core and you couldn't easily re-implement, could make the price high.