Switching from 1.e4 to 1.d4? I've only played 1.e4 as white so far (in clubs, tournaments, online, everywhere!), in the past 5 years since I learned chess. And it has been very fun, but I want to explore the dark side as well, as I do enjoy watching good positional play in GM games.

I'm sure some of you have attempted this before. I'd like to know what your experience was. Was it radically different? Did you experience withdrawal symptoms from not playing 1.e4? Did you have to learn a lot of theory just to get a playable position against well prepared Nimzos/Slavs, etc? 

P.S. I know pretty much no theory in most openings beyond, say 4 or 5 moves.
I don't think 1.d4 itself really makes the game slower; you can use 1.d4 to play aggressive chess. Kasparov and Alekhine did. Likewise, you can play positional chess with 1.e4, e.g. Ruy Lopez, (especially Karpov's handling of it).

So if you want to practice positional chess you don't need to switch to 1.d4, and if you switch to 1.d4 without changing your mindset, you won't necessarily be playing positional chess. 

I first tried 1.d4 after playing for about a 1 year. I was really fascinated with the Qd3 and Bc2 (or Bb1) set up and tried to drive away black knight form f6 and mate on h7, basically my whole plan revolved around that. I didn't find 1.d4 much harder to play or anything like that, but I am sure you are a stronger player than I was back then :).