Need help with wires - Replacing bedroom light fixture with ceiling fan

Hello, first time here and I’m in need of some help. I am trying to replace my simple light fixture in my bedroom with a ceiling fan.
First thing I want to do is take off the original ceiling box and install one that is rated for fans. I was able to install a new light fixture in the kitchen with ease, only 1 set of black, red, and copper wires. However, the bedroom has 2 sets coming from both directions of the box with everything twisted together (black-black, white-white, single red, and copper-copper with a nut grounded to the box). The original light fixture only had 2 wires, black and white. The black wire was connected to the red, and the white wire was joined with the other 2 whites. The black-black and copper were not used?
Am I suppposed to untwist and separate all the wires, knock the box out, then reconnect them again the same way? Why are there 2 sets of wires like this? The bare copper is twisted on pretty hard and tight.

Thanks everyone