For anyone who is unaware, I might as well explain the plots of these shows and how they attempted to stitch them together into a show called Robotech.

Macross: An alien battleship crashes onto Earth and humanity retrofits it with their own technology, naming it the Macross. The aliens show up to take it back and to prevent them from attacking Earth, the Macross warps into deep space along with the civilians who were living near the vessel. On their way back to Earth it is discovered that the aliens, the Zentradi, are gentically engineered super soldiers in service of "protoculture" and almost identical to humans, not to mention that humans may or may not be an offshoot of protoculture. Their weakness turns out to be exposure to any enjoyment outside of battle (in particular music) and they eventually lay down their arms after a devastating attack on earth, so they can build a new future together.

Southern Cross: Humanity has built a colony on another planet, which turns out to be the homeworld of an alien race, the Zor, who have a somewhat shaky grasp on individuality due to having three bodies per individual. The Zor start a war over who has the right to live there. Significant information is exchanged between the sides when a human soldier who was captured by the Zor is returned to the humans. The Zor plan to release spores to "unify" the planet which partially succeeds but the captured human soldier wants to hurt the Zor no matter the cost and severely dampens the effects (the show got canned so it is unclear what would happen next).

MOSPAEDA: An Alien race has conquered the Earth. The Mars colony sends help but only one soldier survives, and it turns out that the aliens, the Inbit, mostly leave humans alone if they aren't attacking. As it turns out the Inbit are just living on Earth because their own world was destroyed and once they are convinced that Mars will retaliate, they decide to leave.

Megazone 23: A teenage boy and his friends are living in 1980's Japan and when they find a weird motorcycle, they are drawn into a conspiracy. Turns out it is not 1980's Japan, it's the 2500's and they are on a spaceship with a fake habitat that just looks like 1980's Japan, it's all being done to prevent people from trying to take control of their lives. The boy swears he'll get this undone (there is more but none of it was used for Robotech).

Now… Robotech erased the idea that humanity may be an offshoot of Protoculture, and instead made it so that "protoculture" is instead the fuel source from MOSPAEDA and the Zor the creators of the Zentradi, even though they still implied the proper story early on so this made no sense. Then they tried to claim this colony planet in Southern Cross is Earth to keep the plot centered on Earth, even if clearly can't be Earth and if the Zor are the replacement for Protoculture, then why they choose to fight themselves instead of sending the Zentradi (who obviously don't show up) is strange because that contradicts their established tactic. Then it is explained that the spores somehow alert the Inbit and their interest in just finding a new home is replaced with wanting… Conflict with the Zor? And the human soldier became a clone of their kind for some bizarre reason I can't understand the purpose of. Then the plot of MOSPAEDA is screwed up because of that... Gross.
But the changes to Megazone 23 might take the cake. In for it to make sense, they flipped the premise. Instead of the 80's habitat being fake and the reality being spaceship, it's the other way around, it takes place on Earth and the space stuff is just some pointless simulation and what is happening is because of the Zor, the result being that it goes nowhere and the plot is rendered entirely pointless. It's so bad that the guy in charge of the translation, who was fine with all the other nonsense, actually disowned it.

If you can't tell, Robotech is garbage and you really are better off just watching the actual shows it's cobbled together from.