how would it be possible to have a change in a single base of DNA, but have the protein not change and be functional?

Answer: The change in single base results in the substitution by a chemically similar amino acid

Explanation:

Whenever a change in one of the bases on the DNA sequence leads to the synthesis of a chemically similar amino acid (e. g glutamate substituted by aspartate, a fellow amino acid with acidic side chain) the protein formed is not altered, and still retains its function. Hence, this type of single base change is known as Silent mutation