Well, Over Expression is the act of adding or changing the promoter at some point that makes the RNA Polymerase generate more mRNA to the Ribosomes to be converted into proteins, This does a very different job than making a new phenotype, this regulates how many copies that this gene is sent to the Ribosomes per visit by a RNA Polymerase, a new phenotype would be changing the gene to do a slightly different function but sometimes you just want more of the protein generated, in these cases you would use Over-expression. Over-expression is much easier to do then make a new phenotype every-time that you wanted a different amount of the protein sent out, one code versus repeating the same code over and over again, just add a promoter that simple, in many ways you are making a new phenotype by over-expression, just one with a amount of mRNA sent out that you control, rather than actually changing the protein structure itself by changing the coding region.
