Hi all,
OK, got it: if your instance of DNN is not in the root at a remote provider, and you are using IIS URL Rewrite, do not clobber the web.config at the root of your site. For some reason I thought I needed to turn off URL Rewrite for the upgrade, and renamed the root web.config. False. Don't do that.
Upgraded via ftp, copying the unzipped upgrade package hierarchically over the original files, then running
http://<the site>/UpgradeWizard.aspx.
While the "wizard" never returned, nor wrote any notification to the browser instance from which it was invoked, it did in fact complete successfully. So the site is now on DNN 6.2.3 WITH a running instance of the Morpheus skin with SkinLab.
Here's how to upgrade a site with Morpheus/Skinlab: upgrade Morpheus/Skinlab FIRST, before attempting the DNN upgrade.
You will find that SkinLab is very slow, but that it runs, on (for example) DNN 5.6.2, after Morpheus/Skinlab has been upgraded.
After DNN has been subsequently upgraded (to in my case DNN 6.2.3), SkinLab runs reasonably well.
So I take back all the mean things I said and thought about Morpheus's author... well... most of them. If you bought the skin from snowcovered/The Store, and can still log on to the account you used to make the purchase, at least as of 9/17/2012 the most recent Morpheus files (dated November 2011) were still there. Google and you will find instructions from the author about upgrading Morpheus.
All the best -- Mike