Can I change from a mortise lock without buying new doors?
There are certainly modern mortise locks which accept pin-tumbler cylinders. Whether any of them will fit in your existing mortise is another question; you may need to enlarge the mortise. You will undoubtedly have to drill for the cylinders; you may need to drill new knob/latch holes as well (though "decorative" brass or chrome plates can hide a multitude of sins).

If you want to switch to a "cylindrical" design (typical modern knobset, mechanism is housed in the knobs and a cross-bore rather than in a mortise), or a rim (surface-mount) lock, you'll either want to leave the old mortise lock in place, or glue in a block cut to exactly fill the space it occupied, since otherwise the empty space would weaken the door. Same issue exists regarding old holes in the door faces; same solution applies.