They were both presented as being Fonzie’s nephews, right? Besides that, I remember Spike as being a prepubescent boy, with much more of a Fonzie-hero-worship thing going on; he wore the same leather jacket, t-shirt and dungarees.
Which Chachi didn’t, and Chachi was older, too. But the show ran for a long time, so maybe he was re-introduced with some explanation for the name change, much like Mike Doonesbury’s baby brother Benjy was later brought back as college student Sal Doonesbury?
Anyway, I’m sure one of y’all will know.