The problem with that is with a 20% down right now, an 800k loan is costing $4500 a month for a mortgage payment. Even if you traded in 450-500k in equity, you still are looking at a 3100.00 a month mortgage payment with today's interest rates. Sticker shock for sure.
Loan amounts will need to reset at 350k to be more in line with today's interest rates. So you do the math, that means a lot of haircuts on sellers' expectations of their home's value in today's market.