Depends on what you have on your bike.
I've been looking into changing the 12 - 25 I have on my Cervelo for an 11 - 32. I can manage a 6 mile 5% average grade with it, as is. But on a longer ride, with multiple climbs and anything more than 5%... my knees scream at me. I need to change my derailleur and my chain to switch my cassette. Currently I have SRAM Force on my bike. I need to move to a mid size derailleur to have room for the larger cassette. So, I'll be putting a mid size SRAM Apex derailleur on.
Veronica