I've read that some Topstone bikes have an odd rear wheel offset, meaning you can't just swap in another wheel. This may be true for all Topstones,, I don't know.

As others have suggested, riding fast on the road is very different than riding off road. Fitness is important, sure,, but it takes time to develop your spin/cadence, your position, and to learn how to ride with a group. Once you get the road tires on, you should start to see gains over time. You can practice your spin and your smoothness by yourself. Maybe even do some intervals.