Source: Lois & Clark: The New Adventures of Superman season 1 (1993-94)
But why WAS Landes replaced? From what Whalin provided, we might think it was because Landes wasn't "hip" enough, not "boy band" material, so to speak. The media from the time 9and the DVD commentary) reports a different reason altogether: Landes looked like Dean Cain too much. Because as you know, TV viewers are terrible at recognizing faces, costumes and context. (Obviously, in the real world, I doubt "looking like Dean Cain" would be a firing offense, never mind calling it a "problem".)
Because it's not like his Jimmy was a total nerd. He had an eye for the ladies (Cat Grant in particular), played comic relief as Perry's "sidekick" (is Lane Smith the best live action Perry? I think he might be), and was generally a good friend to the rest of the cast. So just network morons who give their audience absolutely no credit then. Ah well.