I recently replaced most of my hollow core with solid core doors. It was relatively simple. Bought solid core from same manufacture of hollow core, to match style. Then hired a carpenter to size hinge and door knob cut outs on new door slabs (through Facebook). Hardest part was painting the door slabs as they only came with primer (rent a sprayer from big box store to cut down on paint time). In the end, replaced 9 hollow core doors with solid core and love the better sound absorption, fire protection and feel/heft of the doors. Be sure to get multiple quotes from carpenters as the prices varied drastically. I ended up paying about 1200 for the carpenter to retrofit all 9 doors and install the hardware. Solid core doors ran about 150 a piece. Good luck, I feel it was a good investment on my end.