If switching from Look Keo to Speedplay Zero pedals, is or will there be any affect on bike fit (i.e. stack height, saddle height)?
Yes. Depending on which Keo, you’ll need to drop your saddle 3-5mm. Speedplay lists the stack heights on their website, though I’m not sure they differentiate between all the different versions of the KEO.
I don’t know the exact diff in mm, but since the cleat fits over/around the pedal body rather than on top of it the sole-to-spindle thickness is definitely less, so your leg is effectively shorter.
Is there any advantage or disadvantage with switching from a bike fit perspective? I know I should know this by now, but if you effectively drop your saddle height will you need to also drop your bar height to maintain?
Yes, you should also drop your bars. Generally, lower to the ground = greater stability (lower center of mass), but we are talking 3-5mm here. Occasionally, the changes are more dramatic. I believe there was one team that changed from old LOOKs to Speedplay, which is about 1cm of total difference. Several riders had to get new bikes as a result of that change.
SWWEEETTTT…so I can tell the wife that as a result of switching out the pedals I just had to get a new bike to fit and blame it on you!!! Thanks… 
SWWEEETTTT…so I can tell the wife that as a result of switching out the pedals I just had to get a new bike to fit and blame it on you!!! Thanks…
As someone who know has a wife, and knows how clever they are, I would be careful. She will likely tell you to just keep your old pedals AND your old bike… 