Racial Emulation is a lovely little feat that lets changelings (humanoid shapeshifters) count as the race they have changed into for most things, including taking racial substitution levels for that race. (Another way to do this would be polymorph shenanigans).
What happens when they then change to a different race?
Does this fall under the same banner as prestige classes, where it's generally accepted (if still slightly controversial) that you only need to meet the prerequisites for the first level unless there's a specific rule to the contrary? (Although the way racial substitution level requirements are phrased, you'd still need to be the correct race at the time you took each RSL, not just the first one.)
Or is that not the case, and you revert back to the default class features when not in the correct form? Or get no features at all for that level until you're back in the right shape, because you don't qualify for the RSL features and never took the default features?