The critical measurement to determine if you can reuse the old spokes with the clincher rims (assuming you're going to reuse the old hubs that were used with the tubular rims) is the Effective Rim Diameter (ERD). If the tubular rim and the clincher rim are both in the spoke length calculator databases (like spocalc) and they both have the same ERD, you can reuse the spokes. If the ERD difference between the tubular rims and the clincher rims is more than a millimeter or two, it's unlikely you can reuse the spokes; they'll either be too short or too long (you'll bottom out the spoke threads on the nipples before achieving adequate spoke tension).

If the ERD of one or both rims isn't in the database, you can measure the ERD:



Illustration from Wheelbuilding, 3rd ed. by Roger Musson.