Thats a tough one Sarah!
If he is happy in his cot, then maybe theres no much point in changing it right now?
However, if he is going to be disrupted with the room change, then maybe you should get it over with and change everythin at once - including his bed?

We just put Aidyn in a single bed last weekend, and since then we are spending 4-6 hours a day settling him (this is for his nap and night sleep combined). He refuses to stay in bed without us lying on a mattress next to him, and that is the only way he will go to sleep... so it usually ends up with me or DP falling asleep on the mattress too, because we have to wait SO long for him to drop off!

Anyway, good luck with it, let me know how it goes!
It is exciting moving to big bed, and I think it was a good thing for Aidyn - I think he 'feels' older now iykwim?
We brought him a bedcover that has cute coloured cars and trucks on it from Best & Less for only $20, and he loves it!