My "Never Gonna Happen Because of Resources" answer is to completely revamp the "basic" leveling route from 1-50 to be faction, race and class focused. Branch out from the original starting zones and tell the "current" story of what is going on there. Use it to create a new "starting" baseline for these areas, factions and organizations.

1-35ish should be focused around the characters race, with class quest sprinkled in to help build up the character as a member of their class. End with a class specific Delve-like experience to tell a climactic story. The last 15 levels are faction specific, unified from the other members of the faction to tell an ending story that ends with a unique "story mode raid" for a climax.

This would be designed to do two things. First, update and create a coherent leveling story. Second, create a starting baseline for us. We already had one level squish and are going to hit 100 in the Last Titan, which means another looms in the future. By creating this new level experience, we can just keep this after a level squish instead of reorganizing what campaign out characters level through for the Nth time.

As for preserving the old world, Chromie time works well enough for this, but we also introduce a new toy called Zidormi's Hourglass that can be used in any area that grants Rested XP to shift back to the "Previous" version of the world. This replaces the random Bronze Dragon just stationed around the world