I'm looking at buying a six/13 with the compact SI hollowgram crank and I was wondering how hard it would be to change that into a standard latter on if I wanted to.

Cannondale offer an adaptor to use other crank and you should be fine, please check with your local dealer.

if it's the aluminum crank, cannondale offers a compact and standard spider to switch over.
if it's the carbon crank, the spider is integrated and you'll have to change out the entire drive side of the crankset
or you can cheapen the bike by installing the adapter and using an inferior crankset.