I don't do many weddings, mostly studio stuff, so someone else may have a better answer, but I would think that any switching would cause some sort of 'pop' or other disruption unless you brought the mic volume down first in which case you lose your background sound.
Two other options -
* a podium mic wired to a digital recorder or spare camcorder.
* while the speaker is approaching, do your mic switching, then in post remove the pop and slide some ambient sound over to cover.