Beware, huge bases like that seriously limit the utility of the figures and make it all but impossible to use them in indoor, town or underground settings. While I think round is a viable choice when the diameter of the round base is equal to the edge-length of the square base one is trading out, and thus the round base is smaller, it's almost always a mistake to go the other way. The way you have those now, you're restricted to playing outdoor skirmish encounters. Maybe that's all you do now but it's not necessarily all you'll want to do going forward. Just my two cents.