All the nav/met/planning stuff is common to both so you won't have to do all that again. The mechanics of flying are totally different. An aeroplane wants to fly and you simply have to get it to go where you want, a helicopter is dynamically unstable and you have to actively fly it all the time. It can all go pear shaped in the blink of an eye so your concentration can never be allowed to lapse.
The first 15 hours or so are incredibly difficult as you have to establish the automatic responses in your brain, but once you have mastered hovering it does get a lot easier.
I did the switch about 15 years ago and it was the best thing I ever did.
Go for it.