My oldest boy (3.5) has always had a hard time letting go of his old shoes when it is time for new ones. He really gets emotionally attached to them.
It has been proven time and time again that it is best to ease children into big changes. If your son is in fact emotionally attached to his Crocs, then you don't need to break his heart. Try something like letting HIM choose some cool socks, and let him play with them so they become "his" socks. Then, well, this is what I did to get my son to where his new shoes when he was around 18 months:
I put his old shoes out of sight and then told him we could go for a walk. As I tried to put his new shoes on he wasn't happy. I calmly said he had to wear them to go on a walk. He caved just enough for us to get out the door. Once we got outside he was distracted enough to forget about his shoes. Later, we saw his grandfather and I had him make a big deal about my son's "cool" new shoes. The next time we went out he wanted his cool new shoes.
He felt like he had some control in the matter. He got to choose new shoes, or no walk. Then he felt proud of himself for wearing the knew shoes when my father-in-law made a big deal about them. I hope you can get your son to go along with something like this. Good luck.