What could happen if a mutation in a gene caused a hydrophobic amino acid in a polypeptide to be replaced by a hydrophilic amino acid?

The new hydrophilic amino acid would not form the same interactions with hydrophobic R groups. As a consequence, protein's 3D structure would likely be affected.

Different types of amino acids have different properties and thus form different interactions. For example, hydrophobic amino acids (water-repelent) such as alanine, valine, leucine, isoleucine, proline, phenylalanine, cysteine and methionine. participate in van der Waals type of interactions. They often form the hydrophobic core of the protein which doesn't interact with water surrounding.

On the other hand, hydrophilic amino acids (arginine, asparagine, glutamine, aspartat ...) are polar, can dissolve in the water and can join the hydrogen bond network.