Got it, but transition doesn't work. The next fuel up will show the last odometer reading as 0, with no way to adjust.
The only thing I could figure out to do is to create a dummy fuel up - missed - in which I insert the actual mileage, and then proceed from there.
Switching the settings isn't retroactive. It won't change your previous fillups. It'll only affect those going forward. So, if you've been tracking by Trip Miles up until this point, and switch to Odometer, your first entry will be your 'starting point'.