There are many crossovers in existence, but one of the more rare ones is the crossover involving multiple versions of the same franchise. Usually done for comedic effect, it can be a fun way to show differences between two portrayals of the same concept.
There's quite a huge difference between the 1980s cartoon and the 2000s cartoon, in terms of tone, characters, and art style, and seeing the two combined provides a lot of opportunity for fun. Fortunately, many of the obvious opportunities are taken.
The Darker and Edgier
tone of the 2000s cartoon is quite the difference from the 1980s cartoon that many older viewers are more familiar with. However, the differences are exaggerated. The 1980s characters are portrayed as complete buffoons. While the original cartoon was clearly sillier than the new one (despite, ironically, having a more detailed and realistic art style, especially for the human characters and the world), here all the 1980s Turtles act childish, constantly cracking jokes, never taking anything seriously, and even crying in fear after a fight.
The "cracking jokes" at least makes sense, and is used to point out a fun difference between the original and the newer cartoon: fourth-wall breaking. The 80s Turtles occasionally break the fourth wall and joke to the camera, which at one point confuses and angers the 2000s villains, who wonder "Who are you talking to? There's no-one there!"
One of the more fun jokes is about the different characters in the different TMNT
universes, and their portrayals. Krang (80s cartoon) is surprised that Shredder (2000s cartoon) is some kind of small, cartoony, octopus-like alien with a HUMAN daughter, for instance. The 2000s Turtles are surprised to see that 80s April O'Neil wears a yellow jumpsuit.
While the 2000s cartoon is Darker and Edgier
, the tone of this entire movie is meta and playful. The 2000s Shredder allows his hatred of the Turtles to consume him, and he decides to search the multiverse for the source of the Turtles' existence, which turns out to be the original black-and-white 1983 comic book made by Kevin Eastman and Peter Laird.
Don't expect a great plot from this. The story is just a setup for a fun premise, and to play around with a lot of crossover ideas. Other than the Flanderization
of the 80s characters, it's pretty fun.