Okay, this is odd...
Given Allen's kind offer of help, I created a new user name for him to use, and found that I was able to use *that* name to proceed with the upgrade.php. Bizarre.
Thank you for that Allen, at least I got farther in the upgrade process.
New oddness:
This is a clean install, directly unzipped from the download in the member section. Upgrade.php now reports:
[] The following files do not exist in your install
(Make sure these are uploaded)
--------------------------------------------------------------------------------
All files uploaded.
The following files are reporting a wrong version #
(Make sure you updated these files)
--------------------------------------------------------------------------------
addpost.php
admin/importdata.php
admin/importlive.php
admin/selectprunethreads.php
admin/showuser.php
categories.php
modifypost.php
previewsig.php
sendmessage.php
showflat.php
index.php
index.php
The following files need to be removed
--------------------------------------------------------------------------------
All files tagged for removal have been removed.
[/]
I was taken aback, and just to make sure I re-copied all those clean new files to the directory, but continue to get the error quoted above. The headers inside the files all seem to say version 6.5.1.
Any clues as to what I can do next to proceed with this upgrade? It's very frustrating, especially when you try to start with a clean install.

--Helice