Can retroviral delivery systems "overwrite" genes?
Retroviruses, such as HIV, have a genome that is composed of RNA.  Once inside the host cell, that RNA is reverse-transcribed into DNA; it is that DNA which is inserted into the host genome.  All retroviral genomes are integrated through insertion; there is never any replacement.

Some retroviruses do, however, disrupt normal gene function.  Mouse mammary tumor virus (MMTV) or Human T-lymphotropic viurs (HTLV) will often insert inside or near host genes termed "oncogenes," that, when disrupted, can lead to cancer formation.1,2  Viruses could cause up to 15% of cancers in humans.3  So, no actual genetic replacement, but there can definitely be genetic disruption and a deleterious functional change.

This ability, to insert new genes, has recently been touted as a new way to cure many diseases.  Here's the most recent case, where the researchers used a retroviral backbone to insert a functional gene into some children; i.e., genetic engineering (Side note: Every time you see an article saying how scientists used HIV to cure a disease or something, this is what they mean.  It's akin to calling dental X-rays "giant death lasers that fight cavities.").  With luck we will get better and better at it, but right now it's only successful in limited cases for very specific diseases; massive genetic engineering of humans is still far off (unless you count bone marrow transplants).