I received an ad to build a room addition for $19k. Since hiring
this contractor and signing and agreement the contractor has changed
his ad to read $17k for the EXACT same advertised job. My question is:
Legally, do I have to pay more than he is currently charging?

If you signed a contract at a set price for a set job, you cannot just
reduce the price because he has lowered his current market price.  If
he had raised his current market price and you had a signed contract,
would you have agreed to pay the higher price? Ofcourse not.
This is all assuming there is no "most favored nation clause" in your
signed contract that says you will pay the lowest price this
contractor is offering for this job if he offers different prices to
other people.