The site Pop Focal recently posted some clips from the original unaired pilot of 30 Rock, where Jenna, based partly on Tina Fey’s friend Rachel Dratch, was actually played by Rachel Dratch. After viewing the pilot, one of the changes the network asked for was to have the part recast with a more experienced and attractive actress, so they got Jane Krakowski to play the part. Fey tried to keep Dratch on the show by giving her a series of small guest parts, but that only lasted for the first season.
There’s never been much doubt that this is one of those cases where network meddlers have a point. (Network notes are somewhat more likely to be effective when they’re about big, specific things like whether one actor is right for a part; it’s when they meddle with every individual script, or give vague notes like “can this have more heart?”, that the trouble really happens.) Turning Jenna from an insecure woman whose big break gets taken away from her into a vain, entitled diva is one of the things that makes the premise funny rather than sad, and it also helped lead to the crazy, cartoonish tone that the show successfully adopted by the end of the first season. Everything is a little more realistic early on – including the character of Jack; it’s startling to be reminded of what a plausible executive he originally was, instead of the lovable madman he became – but it probably would have had to stay a little more realistic if the Jenna character, and her relationship with Liz, were still based in reality.
The other actress replaced after the original pilot was the one who played Cerie, whose name escapes me at the moment, but since the character has never done anything on the show in seven years, I don’t think even I can come up with an argument for why this change was a make-or-break decision for the series.