Way to pull in new data from web and leave the old?

I am trying to find a way to have the attached spreadsheet pull in new market data and leave the old. For example, the CBOT provides current market pricing for corn starting in May'13 and ending Dec'15. The months change as time advances. My spreadsheet pulls in the current market data (see Corn CBOT tab) and then summarizes it on the final tab, Commodity Prices 12 mo. period. When I originally put the spreadsheet together I only wanted 12 months of pricing (essentially the next most current month and forward). Now, I'd like to be able to use the past months and the future months.

I might have to hand enter some historical prices to get going but starting now and forward, is here a way I can have the program not overwrite or zero out the previous months pricing on the Summary tab and show the past six onths and all forward months on the Commodity Prices 12 mo period tab?

Commodity Prices swine tpk april 2013.xlsx

Thanks!

Sorry that I was not clear - I have not solved this issue and am looking for help. The nature of the problem shifted a bit so I started another thread because I could not figure out how to delete this one.