I don't think it will be a big deal at all - professors understand that their lab is not the best fit for every single student, plus this guy already said he wasn't sure he could offer you anything. I would accept the offer at your 1st rotation and if your current guy brings it up acknowledge that you will be working in lab #1. Alternately, if you think it would be helpful to your current guy to know you are no longer interested (e.g. if he's turning away other rotaters b/c he thinks he's got you and 2 other guys to choose from) then tell him sooner rather than later. Also, don't slack for the rest of this rotation, it's still a learning experience and you can still pick up skills/knowledge to bring back to lab #1. Good luck!