You shouldn't have to replace the backplane or raid controller; SAS hardware is SATA compatible (the other way around doesn't apply though (e.g. a SATA controller won't work with a SAS drive)).   You will however need drive carriers, which Dell won't sell on their own, so you can either re-use the ones from the SAS drives, buy the smallest/cheapest SATA (or SAS) drive that Dell sells, or see if you can find the drive carriers (with the screws) on ebay or so.
I have (will be receiving soon) a PE2950 that has a SAS backplane (PN610) and a SAS RAID (YF437) card. I'd like to convert it so it can use SATA hard drives and have RAID capabiltiy. is the backplane and the RAID controller the only things i need to change?  or is there more to it? THanks  
PowerEdge 2950 SAS SATA Drive type change