Can I do this on my 10 speed setup?

I'm running 12/27 in the rear. It's always been enough for me, but I have some real doozies planned this year where I'm thinking an 11/32 might really be the ticket.

Assuming I will be able to use the long cage, can I use my 10 speed shifters with a 9 speed cassette and chain, or do I need to get the 10 speed version? Prices I'm seeing for the 10 speed cassettes are pretty high.