That was the plan before the 777 delays but not now. Both 508120 and 508131 were overhauled in autumn 2021 so have years of mileage left. However, 508123 has not been overhauled so will almost certainly be the next unit to go, tying in with the December withdrawal mentioned above.
After 508123, based on the overhaul cycle I expect 508122 to be the next casualty, followed by the units once in the Merseytravel revised livery: 508111, 508115, 508117, 508127, 508128 or 508138.
I was on 508123 sometime last week and there was certainly a few... extra rattles and noises I don't normally hear on other units. Not to suggest anything was wrong, but it give the impression that unit was more tired than others.