Within a few days of my post, Fido responded with a 100% perfect solution:
The $50 GPRS Unlimited bundle was added to your account. It was possible to do so as we have not yet fully converted to our new Billing platform – thankfully.
The usual condition is that customers will be able to preserve a non-existent bundle or price plan under a grandfathered clause, which is now applicable to you.
So a big thank you to the kind folks at Fido for taking pity on a Treo owner, extra bonus points for a quick resolution, and triple bonus points for responding far more rapidly than I have in my own blog. My only excuse is that I’m on vacation, with limited Internet access…an excuse that should be deleted from my repertoire, now that I’ve got ubiquitous access on the Treo!