they have to offer you a role of equal pay and responsibility - if it doesn't exist, i believe they'd have to negotiate with your or discuss redundancy of your role

to be honest, if you can speak to HR, i would ask them what is to happen on your return to work first, say that you're aware of the changes and believe your role has been moved to the east coast - what is to happen to you

let them tell you what will happen re role, responsibilities, and wages, then tell them you'd like a chance to think it over cos you're not returning to YOUR role - and then leave them hanging a day or two, call them back to resign. you might find they offer you an attractive PT package that is suitable.... definitely worth playing the game first - rather than just saying "i've called to resign"...