Remove the costume portal, give players omnishards that they can use at any portal to replace their keys.
Then replace all the season 1 heroes with their costume versions anywhere a season 1 hero existed previously.
After all original Atlantis heroes get their costumes, they are added into the mix.
This way players getting the “old heroes” now feels like something good, instead of like getting kicked in the nuts by someone with steel-toed boots with spikes on the end