Has anyone replaced the batteries in a 20V lithium battery pack?
yes but you got to buy cells that got already those fins attached to them
you can use VTC5 or similar 2000-2600mAh ultra high drain cells
I replaced cells in many power tools, but I usually replace only the bad pair with identical cells.
Currently, the tools I use have batteries with Samsung Qs or Rs between 1500 and 2500mAh, so I use identical ones as replacements.
Best replacement is Sony VTC5A
There are cheaper ones of course
Molicel P26A is a better choice than VTC5A.
Why?
unless its proven their cycle aging is on par with Sony they are a bit better on new cells stats, but realibility counts more
Yes, but currently the P26A Molicel (which is an excellent cell, I grant you) is much more expensive and less common (so its stocks are less important) than the Murata (Me too I have trouble with this name brand, but officially we can no longer call them Sony) VTC5A. The latter is in significant stock almost everywhere and at some unbeatable prices: I saw new and recent stocks with prices lower than € 3 per cell for the VTC5A while find P26A below € 5 per cell (even bought in large quantities (more than 500 units)) remains almost impossible, it is necessary that the productions of P26A increase (so that the demand increases) so that its price is democratized, but at the moment the VTC5A remains the cell the more interesting economically speaking for performance remaining fairly close to the P26A.
€ 5 should be the end consumer price.
Non-E-CIG applications, I think the price is less than € 3 . 
I have seen the test report of P26A.
80% CYCLE LIFE over 700
Both VTC5A and P26A are supplied to the same company, but this company has higher orders for P26A because P26A has the best cycle life.