Yes you have an advantage, but also some disadvantages. Here at my gym we regularly do boxing sparring as well as muay thai and the main differences I've noticed are-
Stance - boxing stance is a lot wider
Guard - Muay thai guard is higher than boxing
Defence - Boxers duck and weave a lot more, whereas you just can't do that the same way in muay thai. One boxer I spar with regularly keeps ducking under my punches and if I'm being aggressive he'll cover up and keep his head down. In muay thai you couldn't do that because of knees. You're going to have to get used to defending you entire body, not just your head and upper torso.
Movement - You'll find the boxers cover a lot greater distance when they move or dodge punches, muay thai is very quick with small movements
Conditioning - This is where you're going to be at the biggest disadvantage. Apart from the additional techniques to learn (elbows, knees, kicks), there is a lot of conditioning that muay thai fighters have to do.
Clinch/grappling - This is *very* technical, and it really shows when someone is inexperienced. As mentioned above, you not only have to watch out for and get used to knees, but be very careful of your opponents elbows. This will probably take you the longest to get good at out of all the additional techniques.
Those are biggest differences I can think of, but many people have made the transition so don't be discouraged. Good luck, and keep us informed of your progress.