Replacing 84" entry door with 80" door
Many suppliers offer standard doors with 32" x 84" and 36" x 84" slabs. See for example these ProVia specs. The best solution is to find a door supplier that will sell and install an 84" door.

Yes, it is possible to reframe for a smaller door but the cost and complexity will go up dramatically. And the end result is unlikely to be architecturally pleasing. To a limited extent we can cover up the shorter framing with molding. But we would be talking about a 6-7" molding: 86" original RO minus 80" new FO. Don't do it. Put the money into a more spacious entry door. Your taller friends will thank you.