Can this joist support be replaced with plywood?
They seem to be 2x8, spanded out to the max,with strong backs added to help the frame.Witch is fine for a ceiling joist,to only handle ceiling load. Now you take the strong backs off weaken ceiling joist frame. Want to add plywood more weight.and you make them floor joist,if any load or a room is added due to span you may be under sized. If 2x10 and short span maybe fine .2x12  16 inches on center ,I would say your fine.  After finding out you have a 20 foot span .2x12  24 on center changes things they are spanded out to the max.and strong backs added to help them . Adding plywood and taking out strong back not a good idea. Not a good spot to store stuff.