Changing cells in an old 18V Ryobi Battery

Hi,

 

I have a question about 18650 cells in 18V Ryobi Li-ion batteries.

 

If a used 18V 4AH battery has six good cells out of ten, but 4 that are bad,
is it safe to replace the bad cells with cells that have more capacity,
making sure however, that all parallel cells have similar capacities?

This means that cells would be replaced in parallel pairs.

 

My thinking is that the more powerful cells would not be used at their full capacity,
however the battery itself would function correctly and safely.

 

Dan