Like haturaya said above, better to change as a set. But if cannot, then just change that one cell with something as close to, or slightly larger capacity, and see how it goes.

You can change the cells to higher capacity. You can solder the connections - clean, flux, and solder fast. I solder mine as I cannot justify using a welder once in a blue moon. Take note soldering will use more physical space.

The only thing you need to find out from the data sheet is what is the discharge current of your existing cell, ie what is the drain current, and than match that with the new cell.

You could also search Youtube for "revive 18650" or "reset CID", and see if you want to try that.

11A discharge current. Just find something more that that

Great! Please post back the result, would like to know as I have a lot of battery tools also

Can you measure from
(black) to (red)
B- to B1
B1 to B2
B2 to B3
B3 to B4
B4 to B+, or poke red at the battery itself

It should be 3.5 V to 4 V as you are measuring each battery individually. If any voltage is not within this range, check that individual battery. You may have to charge the battery individually to get them all to the same voltage, as a BMS could also be without a Balancing function

You can try, but I don't think you will get good results unless the blue battery came from a power tool battery pack. If it came from a computer battery pack, usually the discharge current is low, maybe about 3 A continuous only. So when the robot draws 8 A while working, that battery will fail. I hope I'm making sense, been a long day the brain power used up already

Okay in this case, try first so you know the result

When charging the BMS should be warm, as current is flowing. Only when all batteries are charged will the current drop to zero and then also the heat. I can see that the original batteries were discharged down to about 2.9 V but the new batteries you installed are at full charge. Just let it charge and see if all the battery voltages approach about 4 V. If yes, then you are okay