Evner, The existing backplane will work for both SAS and SATA, but as you stated you will need the 2.5 to 3.5 conversion caddy (part # 9W8C4). Hope this helps, let me know how it goes.  
Hello,    So I'm going to be working on a Dell T620 server that originally came with four 500GB SATA HDDs operating on the S110 controller in RAID0. The plan is to upgrade the server with 4 400GB SAS SSDs running in a RAID5 configuration with a PERC H710. My concern is the hard drive backplane. Will the backplane interface properly with SAS drives or will I need to upgrade the backplane in order to support SAS drives?  Also, the SATA HDD are 3.5 and the SAS SSDs are 2.5. I know that I'll need to get Dell caddies for the 2.5 SSDs. Some caddy suggestions would be appreciative as well. Thank you in advance for your time and support.     - Evner
Upgrading a Dell T620 server from 4x 500GB SATA HDDs running RAID0 on S110 to 
4x 400GB SAS SSDs running RAID5 on H710