I found instructions for upgrading MX records, and followed them as best as I could. They did not precisely fit my situation. I improvised.
Instructions say email should begin within 4-6 hours, but can take up to 48 hours.
So what I should do now is simply wait. Check again tonight or tomorrow morning. If there is still no action, check 1x-2x/day thereafter. If the email has not resumed within 72 hours, then see if I can get professional support.
Or I can just check compulsively every three minutes. That works too.
Westhost support was slightly worse than useless. They not only didn’t fix the problem, they didn’t even understand it, referred me to a different department, and then tried to pass the buck to another company.