Why French heavy cruisers cannot answer Italian battleships

Could France convert some battleship tonnage into heavy-cruiser tonnage?

It could ask, but the exchange would not solve France's central problem because cruisers could not counter Italy's fast battleships and Washington imposed no aggregate cruiser tonnage limit anyway.

French politicians measure naval requirements against Italy, so an Italian capital-ship programme makes battleships the politically decisive answer. Under the Washington system, ships below the cruiser ceiling are not constrained by a cumulative allowance. France therefore gains no extra legal capacity to build cruisers by surrendering battleship tonnage. The trade would weaken the force needed in the Mediterranean without buying a treaty advantage elsewhere.