I'd be leary of mix and match, being uncertain on mismatching a battery cell and getting a cell with the wrong capacity and/or impedance in relation to the rest of the cells in the pack.
If the packs software logic overheats the cell during charging, there's always the possibility of fire (or explosion).
The repacking problem comes when you get plain ended cells and not terminal tabs
The heat from soldering a Li battery can be enough to cause it to go critical.
Li batteries go critical at temperatures from 76 deg C through to 95 deg C.
At this temperature they run away and burst into fire .
Been a couple of people badly burned from this happening .
also the prime reason why you never bolt a Tesla Power wall to the side of your house
If you decide to go off grid you must use a flow cell battery which is wet so can not explode or burst into flames , requires no cooling and can be discharged 100%
But of course the reincarnation of P T Barnam has firmly implanted the name Tesla in every ones mind.
For the same reason, grid sized Li batteries are an idiot idea as they can end up using more power keeping them cool than they are storing
Molten metal and / or flow cells are the absolute best solutions but they don't have the worlds richest man spruking total BS in order to make himself even wealthier at a massive risk & cost to everyone else .
As a side note Musk donated a fleet of Teslas to the climate conference in Glasgow again to bamboozle the world leaders.
However there is only 1 charging port available and the local grid can not handle the amount of power needed to recharge all of them overnight so thay are bringing in 200 diesel generator powered recharging stations to handle the load
Smoke & mirrors ?