Tim,
Here is, how I program the DN and I do not need to switch A to B . DN will make the switch according FS Flight plan and will take me to destination.

1-Turn "ON" DN at O/H panel
2-Turn DN "Auto" mode on the Central stand
3-Load the FS Flight PLan
4-First Leg mileage will appear in "A" DN unit
5-After Takeoff turn toward  first leg course intercept
6-A/P engage / NAV mode "ON"

Airplane will intercept to course. Watch the DN Needle. That is all I do.DN will followup FS Plan until to final destination . :D

Sanal