is there a way to undo solvent joints on waste pipe (dissolve the solvent?)?

I need to replace a 92.5 deg bend on a waste pipe with a dual 92.5 deg bend (rcb104) but the joints are solvent and i dont have the room to get below the join to hacksaw it and replace. is there a way to dissolve the solvent so i can remove the old bend and replace with new, or if i break into the old bend and tear apart will it seperate from the other part or will it likely damage the other part?
Also, can the venting pipe be replaced part way through with a smaller pipe and then back into a larger pipe? The reason I ask is the existing soil pipe is where i wish to place a new bath so would like to get bath flush to wall but cant due to venting pipe. the venting pipe at this point does not carry any soiling waste.
Any help on these two matters would be appreciated.