Hi there Rosie
Just reading your post reminded me of how i felt when my DS was that age, and we were about to make a trip to disneworld in Orlando, i was truly worried 'cause 3hr schedule would habe been pretty demanding, and he also started to want the change to a 4hr schedule (he wasn't hungry at the 3hr mark, he wasn't napping well... ohhh it was a bit messy) so i decided also to go "cold turkey" from 3 to 4 hrs, and he took it rather well, for the first few days, the longer A time was a bit grumpy in the end of each A time, but i would take him for walks wich he loves, so he got distracted, and he started to sleep also 2hr naps, overall it was a blessing to change him to a 4hr schedule!!!
As for the trip, well you can imagine that Disneyworld might be a bit overstimulating for him to acctually take his naps in order, so what i did was just to keep to the feeding schedule, and let him nap whenever he wanted, he did this in his stroller whenever he got sleepy, 'cause even if i tried to get him to nap at his scheduled time, there was way too much activity going around so he got ed off, so he slept while on the stroller whenever he wanted!!! i really tried to relax about this 'cause otherwise, i wasn't going to enjoy the trip. Also if it was his bedtime and we were still out and about, i just changed him to his pj's and made the bedtime on his stroller, and cover it with a dark blanket (he was pretty tired at this point so no fussing, and this only happened a couple of times, 'cause i did try to be at the hotel for his abth and bedtime).
So IMHO, i would suggest to change him to a 4hr schedule this week before you go to your holiday.
Hope this helps and good luck with your trip!!!... let me know how it goes!!!