Star Trek: Enterprise
Enterprise had a remarkable wealth of appearances from past series, not just from the usual pool of guest stars that had already made multiple appearances (I'm looking at you, Vaughn Armstrong and Jeffrey Combs), but a number of actors who had been series regulars.
"Oasis" features one such appearance. The story itself was considered fairly derivative of past episodes, one of which featured the very actor who makes a return engagement for the occasion: Rene Auberjonois.
You may remember the name from the man behind the rubber mask of Odo from Deep Space Nine. The holographic community his character has created for himself in "Oasis" is similar to an experience Odo had in Deep Space Nine's "Shadowplay."
Other past regulars who showed up in roles other than the ones they had previous played included Ethan Philips (who was Neelix in Voyager, now a Ferengi in "Acquisition") and Brent Spiner (who was Data in Next Generation, now an ancestor of the android's creator in a trilogy of episodes beginning with "Borderland").
And because I a big fan of a band named Oasis: