It can be simple or quite difficult, it depends entirely on the pcb and the case and your skills.
Just because the case isn't cut out for those keys, it doesn't mean you necessarily have to cut the keyboard, if you're lucky you may simply get away with removing the switches.
If it still will not fit then it can get complicated.
On a traditional board you need to trace the lines for each CTRL key, IF it's just straight columns and rows you may get away with just snipping it off, provided there's no controller or other components in the way such as controller, power supply, etc...
However, that still doesn't account for the RGB, and that's where it can get dicey. Depending on the routing you'll probably need to add some jumper wires, the real fun though comes when you have to reprogram the RGB system in firmware. Addressable RGB starts counting at the first RGB and works it's way up, so depending on the routing it could get out of sync when doing waves and such. If you only use solid colors, meh, no big deal, but anything else will get messy without redoing the firmware. An alternate method would be to just remote mount that RGB and hide it elsewhere in the case where you can't see it using jumper wires. Personally, I'd remove the switch and see if it fits, if not, I'd relocate the rgb (and test it) then cut the pcb, provided there's nothing else in the way. I'd only deal with the firmware if absolutely necessary. This way you can still use the stock firmware.
But wait, there's more!
Don't forget you're still going to have to figure out how to mount everything and you will also have to figure out the USB connector, odds are it's not going to line up with the case hole.
