I own every drama that aired on The old WB Network on Home Video that has been released on Home Video. I even have HDTV rips of 4 short-lived dramas from the last few years of the Network. They recycled actors ALL THE TIME, especially from "7th Heaven".
Off the Top of my head, actors from CW Shows:
1. Dean from "Supernatural" (Jensen Ackles) was a recurring character on Season 4 of "Smallville" & the last season of "Dawson's Creek"
2. Sam from "Supernatural" (Jared Padalecki) was a regular character on "Gilmore Girls".
3. Stefan from TVD (Paul Wesley) gueststarred on episodes of "Smallville" & "The O.C." and was a recurring character on Season 2 of "Everwood"
4. Damon from TVD (Ian Somerhalder) was a recurring character on Season 3 of "Smallville" & a regular character on the short-lived "Dawson's Creek" spin-off, "
Young Americans".
5. Lucas from "One Tree Hill" (Chad Michael Murray) was a recurring character on Season 5 of "Dawson's Creek" & on the first 2 seasons of "Gilmore Girls".
6. Peyton from "One Tree Hill" (Hilarie Burton) gueststarred on a episode of "Dawson's Creek" that Chad Michael Murray was in & even had a scene with him.
6. Dan from "Gossip Girl" (Penn Badgley) was a regular character on 2 different short-lived dramas in the last 2 years of the network.
7. Kristen Bell from "Veronica Mars" & "Gossip Girl" gueststarred on a episode of "Everwood".
There's a BUNCH more I can remember off the top of my head.