I'd hazard a guess that
RIS is unavailable in 2008. If I remember rightly
RIS was depreciated in 2003 SP2. You have to have
RIS installed and up and running before appying SP2 otherwhys you can't run
WDS in either legacy or mixed mode.
There is an outside chance that a straight upgrade install of 32bit 2008 may retain
WDS mixed/legacy modes but I wouldn't want to try it without a full backup. I doubt very much you'll get it working ubder 64bit 2008R2 since I'm guessing moving from 32bit to 64bit arch requires a bit more than a straight OS upgrade, possibly closer to a complete OS re-install.
Is the
RIS server a DC? or a virtual machine? Personally I'd move any non
RIS functions away to other servers, run it as a VM and keep it as a legacy 2003 R2 server for
RIS only until it's not needed any more.