Tim Shoppa wrote:
I am postulating a full-height quarter-wave 40M vertical that is
switchable (relays halfway up?) to a 20M vertical dipole.

You could use a trap instead of a relay.

Another approach would be to chose a fixed vertical height
to give you a manageable feedpoint impedance on both bands.
For instance, a 25 foot vertical requires an easy to implement
base network on each band. EZNEC says the feedpoint impedances
are 17-j130 on 40m and 200+j320 on 20m, both easily matched
using common network components (or an autotuner).

How would the ground radials used on 40M potentially mess up the use
as a 20M vertical dipole?

Ground proximity detunes the radials so length is only of
minor importance. 40m ground radials work fine on 20m.
--
73, Cecil http://www.w5dxp.com