No, the new owners cannot destroy these graves. When they are located on private property, they have a perpetual character and are outside the scope of the sale (Cass. civ., April 11, 1938). Purchasers are not only prohibited from destroying them, but must also allow you access to them.
Only the next of kin of the deceased may request exhumation of the bodies when they have been buried in a private property (art. R. 2213-40 of the French General Code of Local Authorities).