For some reason, the tips for updating installations of photopost Classifieds for use with vb3 are not listed in the Classifieds section yet. But, by browsing around in other sections of the forum (for ReviewPost and Photopost) I found them
In particular the problem I had was after upgrading my forum to 3.0.3 I could no longer log into my Classifieds forum, and so I could not make any changes to the database.
I found this post
by Omegatron particularly helpful:
It explains how you can use phpMyAdmin or some other tool to modify your classifieds SETTINGS table so item #41, the database type can be changed from "vb2" to "vb3"
Now, I can't fathom why that particular parameter is not included right in config-inc.php next to the database settings, where you can reach it with a text editor, but it isn't! You might just need to get into config-inc anyway to adjust your cookie settings.
So, I hope the five minutes it took to make this post this saves someone the hour and a half I spent researching this.