Paraphrasing or rewriting sentences is nearly always possible from a grammatical or sentence-structure perspective. Depending on how it has been reworded, you may need to be careful to avoid logic errors.
The second example you provided could be subject to a logic error.
As the birthrate declines, the workforce will shrink.
The rewrite you proposed in this case changes the tenses of the verbs.
The lower the birthrate is, the less the workforce will be.
The first example talks about the birthrate (presumably of a given region or country) dropping from a given rate to a lower rate at some unspecified time in the future. Given the rest of the work as context, you state that the decline will result in a smaller workforce.
The second (rewritten) example talks about comparing static conditions at a given point in time. The rewrite may work for the situation you are in, but it would depend on the context of what you are writing.