Protagonist saves the world by killing monsters by transforming into different mecha using disks I'm looking for an old anime which I watched when I was young, during the 90s. As far as I can remember:


The earth is flat in the story
The storyline is about this protagonist trying to save the world by killing monsters by transforming into different mecha? 
The protagonist transform by inserting a disk (?) into a device on his hand
At the end of the first arc, the world isn't restored, and the protagonist had to return all the disks he collected to their original place
Each time a disk is returned, one of the bad dudes will turn back to normal
To make things clearer, it is the protagonist himself transforming
A few potential options, some of these are a shot in the dark hoping some of your information is mixed up:


Monster Rancher - Monsters were born from disks and used to battle other monsters. Not a mecha anime and came out in 1999. Protagonist doesn't transform most of the time.
Saint Seiya - Has transforming protagonists that look mecha-like. I don't recall any disks though, it came out in the late 80s.
Escaflowne - Came out in the 90s, mecha like.


It could also be some flavor of Digimon, Gundam, etc.

Very few Mecha anime that weren't Gundam or Macross came out in the 90s that I'm aware of; but here is a list.