For info, I did this is last summer: bought a low miles late 2019 13.8KWh pack from a breakers for £1250 (U.K.). Removed all the modules from old and new packs, keeping note of the old CMU locations. Swapped in the LEV46's into the early casing and put the original (12KWh) CMU's back in their original places.
No issues at all: even the guessometer has gradually come up a little, although I usually do better than it's thinking. Real world range has gone from 18-20 miles taking it easy, then 13-14 miles at best when a cell was failing to a healthy 25-28 miles on the 'new' pack. My car is 2015 with 170,000 miles so the old cells had done well. I have four full 12KWh type modules with the 13.8 (2019) CMU's here if of use to anyone by the way. I'm going to break the last one with failed cell into re-packed units for my electric outboard.
Note, early casing just unbolts (pre-soak if rusty as quite fragile) but the later one is has bonded halves and is most unpleasant to split! In the end I resorted to a blade and levered, sliced, levered, sliced all the way round. The electrics are well inside so you're not going to catch them is sensible.
