I certainly hope the FAA gets their program together and devises a reasonable solution to this...and sticks with it, so people can plan their careers.
There is no way to know the right answer on this. I SUSPECT that if the FAA makes a significant change to the rules which disqualifies folks, then there will be a backlash, political involvement, and a grandfather process established for those who had previously held medicals.
If I were in your shoes, and were committed to flying I would:
1) Get all of my ratings as quickly as possible.
2) Get a first class medical and then DO NOT let it lapse for any reason until this is all over with. Don't even let it go to a second class.
I'm pretty sure they will end up having to grandfather those who were current when the rule changed.