Will learning about EV3 transfer to Lego RCX software/hardware? If I attend a workshop on EV3 will that knowledge help me get up and running with RCX software and Lego Mindstorm kits we already have at our school? I know EV3 is backwards compatible with NXT but they said nothing about RCX. Guess it's a bit obsolete?
As far as the standard software is concerned, it's probably way too different so that your knowledge of one won't translate easily to the other. If the workshop uses third-party software, though, the gap might be smaller.

As for the construction of a robot and the principles of how to automate it, this should be closer, but there are still noticeable differences, for example using studless beams in newer kits versus studded construction in the RCX.

There is some backward compatibility, mind you, but it's more on the hardware level (meaning with the aid of converter cables you can use RCX motors and sensors with an NXT and that should hold true for EV3 as well).