This frequently happens with me re-watching older movies, although sometimes I recognize the name, not the face.
The classic one that hit a lot of people was re-watching the Woody Allen flick Bananas and suddenly realizing that the subway punk who mugs him is played by Sylvester Stallone. Before Rocky, I don’t think most people took any notice of Stallone, but afterwards he was iconic, so that moment of seeing him and not knowing who he was when you first saw the film (if you had seen it before) comes as a shock.
I know that I hadn’t really noticed Liam Neeson before Darkman (I suspect more people first started to notice him a couple of years later when he played Oskar Schindler in Schindler’s List). But then I started to notice him in other films I’d seen earlier – The Mission, The Bounty, Krull.
My most recent encounter with this phenomenon was when I re-watched the movie Becket for the first time in many years, and saw the name Sian Phillips*. She played Thomas Becket’s mistress Gwendolen (yes, that’s how they spelled it). But I knew that name – she was Reverend Mother Gaius Helen Mohaim in the David Lynch version of Dune! I had to go back and check, but it was indeed her.
Of course, I should have known her from other roles – she was Livia in the BBC miniseries I, Claudius ** and Cassiopeia in the Harryhausen film Clash of the Titans, but give me credit for identifying her in at least one other role, even if only by name.
*According to Wikipedia, her real name is Dame Jane Elizabeth Ailwên Phillips, DBE. I wouldn’t have known.
** I, Claudius is a gold mine for this thread. You can see Patrick Stewart with hair, John Rhys-Davies without facial hair, and John Hurt as Caligula, in addition to Derek Jacobi as Claudius himself.