Considering the majority of Telly's scenes in his first appearance have him looking closely at the TV screen, at a sideview, it seems like a waste to give him the spiral eyes and effects. Maybe if we saw the back of the TV instead (which is what I expected... I'm surprised that we got to actually see the TV screen), allowing us to see a front view of his face more.
I'm guessing that Brian Meehl took over as Telly in the eleventh season. I once e-mailed him (he has an official site), and he told me that he began performing in the latter half of season 10, so it's possible that he took over in season 10. Meehl has said that he was on the set when a Telly scene was shot (and it recently hit me that he was probably performing Barkley, who had a scene with Telly in his debut).
Somebody at Muppet Wiki said that the CTW Archives has a program guide for one season ten episode that includes Telly, saying that Telly and Bob do exercises. Makes me wonder if that's the Telly we're more familiar with, or if maybe they were watching an exercise program (among other possibilities).
I find it a little interesting that, when Caroll Spinney was unable to do a show that was scheduled to be taped, that they needed to do it on that day so badly that they just gave Telly a new personality and performer and put him in place of Big Bird. I know, we probably wouldn't have the Telly we know today (and might not have even seen any footage of the original Telly... Would they have included a clip in 40 Years of Sunny Days and/or included the whole episode on iTunes if Telly didn't become a major character?), but it's interesting they did that instead of deciding to just reusing street scenes from a previous season (maybe doing the episode at a later date). Then again, "time is money", and they might have decided to go through with it since others were on the set (they probably wouldn't want to have to pay the cast and crew for what little time they were there for if they got to leave early because they didn't do an episode). But considering Brian Meehl was on the set (he was reportedly the only puppeteer on the set that day), they must have planned for him to do some segments.
Brian Meehl did inform somebody from Muppet Wiki who contacted him that after the original incarnation of the character was dropped, they used him as a utility monster for a short while (which explains why the puppet was used as the Two-Headed Monster's mother) before he gave the character his new personality. I'm guessing this happened in season eleven, and he became a much more major character in season twelve. There is a season 12 press release that refers to him as a new character, which could have been referring to him as a main character (though that same press release also refers to other characters known to have been around before as "new"). I doubt his sole segment in episode 1446 was what was intended for Big Bird... And the next episode did involve Telly, but also had other Muppets and performers besides Brian Meehl (who said he got the role because he was the only performer on set).
We probably won't know what episode was the one where Telly was first performed by Meehl, unless we get access to every episode from seasons 10 and 11 (or at least every episode between episodes 1257 and whatever is the first to clearly feature Telly outside of his television obsession), or maybe if somebody finds guides to all those episodes. Then again, I wonder if Brian Meehl remembers what happened in that episode.