The Larsen 6dBd Yagi antenna has been FCC-approved for use with the RF451. I'm curious if I can swap out a 6dBd omni and still not exceed the 1W threshold?
Yes that should not be an issue. Swapping out the yagi for a omni just changes the pattern of the radiated and received signals. Instead of your +6db signal being sent in one direction, it will now be radiated in a circle pattern out from the antenna.