Paint them. What color will all of your trim be? We owned a rental property ( double house) for 27 years. The front doors were exactly like the one you posted!! It was very sturdy and worked perfectly...so we kept them. Oh how I hated that look!!

If you are having white trim I would paint all of the doors white.

You can add trim detail to both sides and paint them, or you can replace doors by having distributor pre cut doors for locks and hinges to match your existing frames, all you have to do is give them 3 dimensions. That will save you time and labor and when new doors arrive, all you have to do is hang them onto the frames.

Paint white and add molding to every door except the front door. For the front door - replace it. Video below is very informative.

http://www.ronhazelton.com/projects/how_to_make_a_flat_door_into_a_panel_door

There's also this idea. http://www.remodelaholic.com/5-panel-door-flat-hollow-core-door/