iPhone shows wrong location
It turns out the only thing to do is wait. I called Apple and the rep said that if power cycling the phone and "forgetting" the wifi network didn't help, the Apple database would clear stale entries after 30 to 60 days. Sure enough, after about a month my phone started showing the new location.

(Note: I tried @bmike's procedure several times on each of the iPhones in our household, but it didn't seem to make a difference. It could be that the procedure does remove that phone's historical location info from Apple's database, but over the many years we were living at our old address we had enough guests with iPhones that removing only our own phones' records wasn't sufficient. If anyone else is having this problem you might as well try that procedure, but know that the problem will self-resolve within a month or so.)