Well, no, you can't "convert" to BB-30 from English. However, I think you probably have a BB30 frame it sounds like, with an adapter installed to allow you to use English. If you remove this adapter, it will be ready for a BB30 crankset.

The problem is that the adapter is glued and pressed in. Some manufacturers don't have a problem with you removing it, some do.

I have heard that Cannondale doesn't like you doing it with their carbon frames, and I think it voids your warranty.

It's up to you if it's worth the risk. Get a good shop to do it though if you decide to try it.