Well, see how the pm nap goes, ie if he wakes earlier but seems refreshed, like he's had plenty of sleep, then you may want to shave a little off the am nap the next day, or extend his A before the pm nap instead. But if he wakes earlier and really isn't happy about it and you feel he's OT, then you can try resettling and iiwm, I'd probably try the same again the next day, since it would take me a few days to know for sure that it's OT waking him because I was pushing him too hard, rather than him waking for some other reason. It can take a while for them to adjust to sleeping a bit longer in the afternoon - same applies when going to one nap too.