3/8 is pretty common. If you have some stubborn existing threads you can try heating them up and than trying to knock them free. If you are unsuccessful you can usually take out the nature thread.
Funtronics carries both the threaded inserts which can be spot welded back in place and than just simply reinstall the casters of your choice!!!
Here is in old thread when I replaced both the inserts and the legs on a Vendo 56.
http://soda-machines.com/discussions/index.php/topic,16896.0.htmlJared