Can I put clincher tires on tubular rims? Are all tubular rims unable to accept clinchers?

I have two older bikes, one is a Bo from the eighties that has Mavic rims with tubulars and the other is a Motobecane from the seventies with alloy rims. They both are 700C rims. They both seem to have high sides but I haven't taken either apart, but I have to as the tires on both are aged and need replacing. I'd like to use clinchers for the ease and economy. Are there parameters to make this call or is this change completely out of the question without changing the rims totally?