How complicated is it to move a laundry room upstairs-there is a bathroom in the room right beside the new laundry...
If I later decide to change it back to a bedroom, how can i "hide" the waterline/vent hole?

1) not complicated if the washer is going to be on the other side of the wall where the bathroom sink is. assuming there is a vent on the drain line or the stack is there, and you have 1-1/2 to 2 inch drainage possible.

2) you hide holes by putting caps on the water/sewer lines as close to the risers as possible, then patch the wallboard and fix the floor.

3) if it isn't code in all areas, it is a bloody good idea to have the washer on a drain system, and a floor drain in the laundry room, as if it was a commercial laundry. one busted pipe can wreck almost all the walls and flooring (and contents in affected areas) in about an hour of water flowing.

4) your modern front-loading washer spins at NASA speed, and it shakes things real good and makes lots of noise. I know a fair number of folks who moved the laundry room upstairs, and then after trying several different washers and retrofetting in additional reinforcement, moved them back into the basement.