In an early episode after Zordon leaves, the Rangers (sans Tommy) are captured and apparently have their powers stolen from them to create the Shadow Rangers. Tommy goes to Dimitria for help, but instead is forced to solve the problem himself with Dimtria only asking questions they already both know the answer to, yet still leading him in the right direction. This shows that Tommy and by extension the other veteran Rangers, no longer need a mentor to help them solve problems, but they can do it themselves instead. This set the stage for Tommy to become the mentor (albeit years later) in Dino Thunder.
Not surprisingly the episode was written by Judd Lynn, who was responsible for the series Growing the Beard.
Divatox's initial motivation is to avenge Maligore by killing the rangers, so it seems weird at first when she continues to attack the rangers after the cast change, since four of the five responsible for Maligore's death aren't even rangers anymore. Then you remember that she sees the Millennium Message shortly before the changeover occurs: she sees that this new team will stand in the way of the United Alliance of Evil's conquest, so her priority shifts to eliminating this threat.
In the movie, the Rangers never thought to have Lerigot heal Rocky's back the same way he healed Kat's leg. It would have been much easier to do this than to try and squeeze in a last minute replacement who has neither the experience in the field nor the rapport with the team that Rocky does.
Justin being able to morph directly contradicts one of the few rules of the Morphing Grid previously established by Zordon, in that he is a kid and not a teenager. During the third season of MMPR, Master Vile turns the Rangers into children, because in that form, they're unable to morph. That's what necessitated the use of the Zeo Crystal to turn the world back to normal, so they could become teenagers again and thus regain the ability to morph. As ALL of the morphers are connected to the same Morphing Grid, it doesn't make sense that the Turbo Morpher should be any different than the Power Morphers by allowing a kid to become a Power Ranger.
I remember Vile's plan was to turn the earth back ten years, before the Rangers got their powers. That's why they needed to find the Zeo Crystal.
As the Troper above me stated it was because it was before they got their powers that they couldn't morph, however, the real problem is that Zordon has repeatedly refused to grant de-aged Rangers new powers because they're too young to be in such danger. So, why is it okay for Justin to be in such danger?
I think the reason he didn't want to endanger them is because if they were hurt in the past they wouldn't be able to do all the good that they do in the future.
TJ initially tells Cassie that he wanted to be a Power Ranger "ever since [he] was a little kid". Yet the Rangers have only been active since 1993, while Turbo is happening in '97. Meanwhile, TJ's got to be at least half done with high school at such a point (yes, the new Turbo team was still in high school), meaning he would be maybe 12 or 13 when the first Power Rangers were active; hardly a "little kid".
To be fair, time has been reversed repeatedly during that period, and there were rangers active during that time so he'd remember it as him still being a kid. Bit of Fridge Horror there seeing as havinbg new memories inserted into your childhood could drastically change the kind of person you are.