I just redid one of the 3S packs I built out of old SMC 2S 7200mah packs yesterday. Had a cell drop in it. I had one more old 7200mah 2S pack to sacrifice, so I cut it down, separated the cells, then took the bad cell out of the old one to replace with a new one. However one of the tabs for one of the paired cells on the remaining good ones that I had to desolder wouldn't take solder again... no matter what I did. So, ended up removing 2 pairs of cells and resoldered/installed both from the sacrificial pack.
I use typical masking tape rolled up around a q-tip and squeezed to make hard backer blocks for the tabs to give them something to rest on. Then used 4-5 layers of paper masking tape over each of the leads. I don't have any of that insulating tape, other than vinyl electrical tape, which is soft.
Kind of unnerving to be fiddling with something that has so much power in it. One wrong move with a knife and I have an instant fireball of rage on my workbench in my apartment. SMC uses some sort of glue to bond the pairs of cells together, which you have to carefully cut to separate them. I use a plastic putty knife for that.
This is from when I built them the first time, I made 2 3S out of 3 2S packs:
I bought 3S balance plugs with silicone leads to use. Wish they were about an inch longer.
Then used the case halves as some sort of protection for them after wrapping the cells with masking tape over the leads and fiber tape a couple layers to hold the cells together, then a couple layers on the outside of the case halves:
The one in the middle is a hobbystar 3S 5400mah pack. I had 2 of those to try out 3S... one of which has since dropped a cell. It's 3 months or so sold with maybe 8 cycles on it.
Getting the case off is tedious. I used a dremel with a cutting disk and very light pressure to work around the seam until I see a glint of the silver pack underneath.