The house had a mix of interior doors when we bought it. Some 6-panel doors downstairs and these flat doors upstairs.
For about a year after we bought the house we debated buying all new interiors doors or just replacing all of the doors upstairs to 6-panel doors. Although there are a ton of awesome door styles available we decided to stick with the 6-panel door. Lowe's runs a promotion every few months where they offer a free upgrade to a solid core door (instead of the standard hollow doors) which is about a $30 savings per door. We took all of the measurements, waited for the sale, and ordered 7 new doors. We were told that the doors would be easy to hang by routing out the locations of the existing hinges and drilling a new hole for the door knob.
RIGHT! This was NOT an easy job (which is probably why I don't have any pictures of the process)!. It consisted of tons of measuring, routing, drilling, carry doors (remember the solid core = heavy door) up 2 flights of stairs and then back down to the basement, painting doors, and installing new hardware. It was a worthwhile improvement but I know Andy had many moments when he wished we would have bought all new pre-hung doors and just installed all new doors, jams, and trim. That would have been a lot of work too, just maybe not as tedious.
Ignore the old brass handle on the metal bi-fold door. This is in the spare bedroom and someday those doors will be replaced.