As has been said earlier, stay well away from this and just replace the units with new if you know what you are doing and certified.
Better still leave it to the professional fitter who has all the certification or go and get trained.
I have seen over the years some really badly fitted replacement units.
Not packed out with spacers, incorrectly packed out, wrong glass fitted when it should have been safety glass, and that was in a set of french doors if the young children of the family had run into the glass, well you can imagine, the list goes on.