Substituting a complex embedding for a number field element

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Is there a way to take an element of a number field (or a polynomial or a number field) and replace the generator of the field by one of its complex embeddings? For example, something like

sage: K.<a> = NumberField(x^2 - 3)
sage: (3*a + 5).substitute(a=a.complex_embeddings()[1])

It's feasible to do this with sage_eval, but hopefully there is a better way.