The ATPLs are valid for 3 years from your last pass. During this time you have to get both a CPL and an IR if you only get one then after the expiry they revert to the relevant exams for that one ie if you get a CPL only after 3 yeas they revert to just CPL exams. Also if you let them expire with only having obtained one of either a CPL or an IR there is no 'upgrade available back to ATPLs so you would still have to do them all again to get back the ATPL credits.
Once you have a CPL and an IR your ATPL exams are safe as long as you do not let your IR lapse by more than 7 years. It is only once you have your full ATPL issued (ie 1500 hours 500hrs multi crew etc) that the exams are safe.
Hope this clears it up, unfortunatley it really is this complicated.