Hi, I am for GPL (I am just a user, not a contributor, and not a lawyer either) - I think GPL makes it more likely that a project like this will continue as free software in the long term, which is good. I will sum up my thoughts about the change, although most of it is in bits and pieces in previous messages.
- Any existing user or developer (even commercial one) who acquired the code under MIT would not be affected at all to use it, because he got the code legally under MIT licence and this licence grants him the rights it grants forever. If the licence is changed by the developers/owners, I think the existing code is at that time available under both licences simultaneously, and the new pieces and updates only under the new one.
- As there is a community of developers, there should be an agreement among the existing contributors to do the change. Maybe to get a formal unanimate agreement more easily, it might be enough to ask all contributors first if they would agree to the change, if the majority supports it and then organise an online voting - just an idea, I do not really know if this would suffice all formal requirements.
- I really do not know what happens if there is no response regarding agreement with the licence change from a contributor, who is not active anymore, changed his/her contacts, etc... I think that would be a problem.
- If there are existing contributors from the commercial sphere, they would need to either adopt GPL, or they would probably stop contributing, and will continue to develop their own code under their own licence from the versions acquired under MIT. I don't think it should be a major problem for them or for the project (if they as contributors do not block the change, that is).
- Surely there are some other projects which have undergone similar changes in the past. If someone knows of one, you may just ask their community how it was done and how it went.