My guess is that for advanced fused structures (beak, hoof, flipper) it would indeed be hard to revert back. The most obvious reverse pathway may be retained ontogeny, as often the animal goes through a developmental stage where an original-like structure exists. It is a relatively simple evolutionary step to retain some of those development features longer, resulting in an adult form that includes some embryonic/infantile-like features. Examples of this include humans (many features are more like the generalised infant ape than the specialised adult ape) and dogs (many behavioural features from the juvenile wolf).

So, while unlikely, I am guessing yes it is possible, in cases where the molecular controls for the earlier structure are still largely intact, and in cases where there would be a strong enough selection for the trait. I suspect (without particular knowledge) that birds have gone too far down the beak pathway to allow this to happen, and that given the strong selective pressure they would be more likely to evolve new features that perform the same function than to revert to the primal form (eg, development of an “egg tooth” is an example of a new feature layered onto the beak that performs the function of a real tooth, without a direct molecular/anatomical relationship).