I built a lot of rods in my youth (40-50 years ago.) Never converted one. But some info from building might be helpful. Old rods with thread wrapped guides sealed with varnish should be pretty easy to convert but the newer ones with epoxy attached guides might be a real bear to remove down to the blank. You also generally need more guides on a caster than a spinning rod to keep the line off the blank in a bend. Also, even back in the day a casting rod blank was laid up differently to a spinning rod blank, so it may not get you where you want to go.
As for the handle and seat, spinning rods were usually made with the blank going all the way through the seat whereas casting rods (without cork) were more frequently glued into a socket in the handle. If that's the case you'll shorten the rod a bit when converting.
Rods have a spine. Spinning guides go opposite the spine and casting guides go on the spine. So your spinning guides need to go on the opposite side of the old guides for best performance--assuming the original builder put the original guides on right. Thus you have to be able to rotate the seat as well. Way back when I could feel when the guides weren't right. Don't know if I could today--I went too many years without being on the water and now I'm just occasional hack out there.