Yes you can convert.
Best way is first to invest in one of these .. my favourite tool of the moment.
http://www.ebay.co.uk...M&hash=item5ae0d94633
Cut out sash recesses in frame .. and as builder says .. fit new casements into it.
It's important to emphasis about the squareness of the openings tho.
Many sashes seem to go out of square on old buildings. Weakness in their design I think.
Personally, I would replace with modern tilt sash replacements .. so they look the same!
Al.