If it's already leafed out, I'd think you could slip-pot to a slightly larger pot if needed, but probably should leave major root work till next spring. I have limited experience with these...one nursery plant purchased in fall 2011. Last spring (2012) I removed about 1/2 of the root mass, pruned heavily and repotted. It produced strong growth. I had heard about the "1 growth flush per year" and tested by pruning back some of the stronger new shoots...no new growth after that, which supports the idea. This spring I repotted and removed more roots, looking to expose surface roots...didn't find much, unfortunately. Be prepared, these things produce very dense root masses.
Euonymus are pretty tough. I have one that is planted in the garden. Two winters ago voles removed all the bark (as far as I could tell) but the plant is still alive and growing. So you might be able to get away with root work and hard pruning now, but why chance it?
Chris