You are correct, the unit can be replaced without the frame but:
Sometimes in old windows the seal from the frame will have adhered to the glass making it difficult to get the glass out cleanly. In addition some of th older cheaper units had poor quality uPVC that coupled with the above mean the strips you take out can snap.
To get the strips out use a thin knife such as a putty knife. You will probably need to get the unit out to measure the thickness as they do vary.
In addition K glass now has a 'black' look so replacing one pane may look odd.
Note if you have the older frames sometimes the glass unit is taken out from the outside. It is worth considering replacing the frame as it takes less than a minute to get one of these out (especially if you dont care) so are very attractive to burglars.