Have you tried going to the data source tab and clicking refresh (it works a little differently than right clicking the data source and refreshing the extract)? It will definitely update with the new dimensions / measures if it's Excel, and might work for server based data sources like SQL server. If that doesn't work, then the only other work around i can think of is to remove the extract (which will make it live and grab the new columns), and re-extract
Hmm I'll have to try it that way when we make some changes. The only way I've tried is right clicking the data source.