I think we're talking a specific over general situation here.So the Lorwyn Changeling is broken. As in, it has a feature that doesn't work at all.
It gets a feature that lets it take the Dash action as a reaction when you roll initiative. The Dash action grants you extra movement on the turn it is used. But you can't use that movement outside of your own turn, so the feature doesn't do anything at all except waste your reaction.
This is glaring, because every other time in the history of the edition, the designers have understood that they need to explicitly confer the ability to move when a feature lets a creature move outside of its normal movement. By trying to use the Dash action as a shortcut (most likely to encourage optimizing around this feature), they've made a feature that's entirely non-functional as written. (Even if your character gets the top initiative roll, there is no current turn for the benefit to apply to before the actual first turn anyway.)
The rule for the Changeling.
I read that as an immediate opportunity, prior to anyone else's Action, to Dash.When you roll Initiative and don’t have Disadvantage on that roll, you can take the Dash action as a Reaction.