As long as you have some form of IR current (SPA/MPA-SE or ME etc.) within the last 7 years, your ATPL theory remains valid.
In reference to the SPA ME/IR, If you allow your IR to lapse by 5 years you will only have to retake the flight test with a CAAFU examiner again.
If you allow your IR to lapse by 7 years, you would be left with a CPL (as long as you have had it re-issued of course). If you wanted an ATPL you would have to retake the exams or if you just wanted your IR back you could just do the IR theory. After the flight test that would give you a CPL/IR again but you wont be able to upgrade to an ATPL without resitting the 13 exams (no VFR comms needed).
Do you have to resit a full ground course or can you just sit the exams again?
I think you can just sit the exams again but dont quote me. If I remember correctly the book says "training as required" Head of Trainings discretion.