Not sure I buy the idea that Yoda would only be a Knight during the time frame of the movie. In Episode V, Yoda says to Luke, "For eight hundred years have I trained Jedi." The idea that he'd have been training Jedi for over three centuries and still only be a Knight seems unlikely, even though there's no canon source that says exactly when Yoda became a Jedi Master. I mean, I suppose he could have fallen foul of some nasty tenure arguments at the Jedi Academy on Coruscant and had to wait a long time to get tracked, but that doesn't seem likely.
Also, while I'm not the biggest Star Wars fan or lore guy out there, I can't say I've ever heard of the 'High Republic' era before. If they just want to say that the 'High Republic' era is 400 years before the prequels, that's cool and all, and it's not like there's a whole lot of other activity going on at that time that's already been established (the only thing I could find with a quick Google search is that Yaddle, a character of the same species as Yoda who was on the Jedi High Council during 'The Phantom Menace', was born in 509 BBY (at least according to the Star Wars Character Encyclopedia), and would arguably make a much better 'young Yoda-like character' to be a main character).
I feel like the time-frame is probably less important than getting the story away from the 'legendary' original series characters and established with new characters and stories. In a way, it's oddly similar to Star Trek, in that the TV series have been fine with going in different directions and pursuing different stories, but the movies are so fixated on telling you all about James T. Kirk that they just can't pick up any momentum in any other direction.
Time will tell, I guess.
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Pauper