When you build a new RC, the type and quality is random. There is no way to improve or modify an existing RC. It is what it is. If you don't like it, you tear it down and build another one.
The lord can choose to specifically build an infantry, archery, or cavalry center, but there is a chance that it will fail, and you end up with a (literally) useless center.
The only way to get an MI or SF center is by building a random center, and getting lucky.