Hi,

I believe that a conservative replacement a mutation where an amino acid is replaced by one with similar properties (ie Glutamic acid being replaced with Aspartic acid), as opposed to a radical substitution where the amino acid is significantly chemically different.

However, a complementary substitution would refer to a pair of amino acids which had both mutated into a relationship that was functionally the same, but with different amino acids, such as in this ionic pair:

Asp Lys going to Lys Asp.

I would have to see the context to know for sure though.