I'm having that exact problem!
Only I DIDN'T accidentally make a duplicate of my original version of Toast, so now I can't use it AT ALL. They can retrieve the number for you IF you have the email address you had then, and the last 4 digits of the credit card used to buy it. I've had it YEARS so I can't even guess what THOSE would have been at the time.
I saw a couple of ancient software boxes last time I was in my storage unit, but I'm almost certain that was followed with .... "Huh? Why would I still need THESE?" and throwing them away.

I'm not even sure Toast was one of them. I'm going to re-check there, and some random data backup discs, in case my original Toast is on them, but that's all I've got. Could you message me if you find a solution that DOES allow us to update?