I believe Mark VI Blue was the first to arrive at WDW in 1989- but it wasn't until 1991 that the fleet of Mark VI monorails was in regular service with the last of the trains being Coral but I don't remember when that arrived and went into service.

There was quite a bit of work required to upgrade the power systems to support the operation of the larger Mark VI trains so there was a while when the Mark IV trains ran alongside Mark VI trains until all modifications could be completed to support the full set.