why do you think silver wears different boots in sonic riders zg and free riders

Maybe it's because of his powers? Maybe his powers affected the gravity rings in Sonic Riders ZG, and in Free riders he uses those boots to stay on the board, maybe?
Other than that I don't know.

Mabye because he (like every other sonic character included in the series,with the exception of eggman and metal sonic) recieved a desighn upgrade to keep up with the futuristic look of the rider series.....

"I think its because he's just getting into the wonderful world of wearing mais clothes...."