If they need servicing i.e. sash cords, freeing off etc then I'd take
them out and do everything you can to them inside the room including
paint preparation primer and undercoat. Then re-hang and adjust etc,
if necessary make good any marks to paintwork, then finish off in situ
with gloss, ladder of necessary.
If they don't need servicing - old paintwork is best left on if
possible. You could smooth it by sanding - 60 grit and cork block.
Wash with sugar soap and paint as necessary in situ.
In other words final paintwork is best done in situ as gloss takes
some time to harden and will get damaged if bashed about a bit.