you can play hardball including taking them to court for age/sex/etc. discrimination.
Wtf?
This highlights what is wrong with so many people today.
Taking someone to court for discrimination is only going to happen if there is a case to be made and there is nothing wrong with litigating a case that can be made.
You are a successful young man, you've probably never been on the receiving end of "I can treat you worse than other people because you are a woman/old/disabled/a person of colour" and know that nothing is going to be done about it because the only people who "matter" are successful young men who don't care.
In this case my advice to OP would be to not sign anything at the moment and just keep turning up for work on your current 60% schedule. Let your managers do all the work to make any change happen. If any discussions happen you should be asking -
(a) why as both roles A and B are now going to be full time why you are being moved out of role A when all your reviews demonstrate that you are performing well in the role,
(b) who are they proposing will fill the role that you are not in,
(c) whether the full time pay in role A will be the same as the pay in full time role B,
(d) why the pay in either or both of these roles (whichever it is) is effectively being reduced by such a significant amount and whether the pay of people in other roles is being reduced through making them full time or only yours.
And document everything in real time. Make sure you have all your personal documentation (employment contract, reviews, etc) at home and if you can legally take eg organograms of your department as evidence of who does what and their pay and status do that too.
The aim here, I am afraid to say, is not necessarily going to be for you to either keep your job as it is or get better pay in the new job. It's to make life as difficult as possible for your managers while appearing to be entirely reasonable, and to get as much information out of them as possible which you can then use as leverage to get dismissed with a settlement/right to unemployment rather than resign or be forced out without those things.