Hi Penny, I certainly have had hiccups when upgrading before. Most of the time, in the past, it has been due to third party modules or me doing something I shouldn't have done. I do know that when I went from DNN 4.9.x on some sites to 5.x I had issues because I wasn't using the sa account for my connection string (I usually use an account specific to that database, not the built in SQL Server administrator account). When I attempted the upgrade again using an sa account (for SQL) it went off without a hitch. 

While I don't know how you cannot go through the process again, what I would do this time around (this is basically my upgrade test process for high volume and/or highly custom dnn sites):

• Restore a copy of the site and database locally
  • Make sure you add your localhost/virdir portal alias in the PortalAlias table
  • Run the site once locally, making sure you can use it locally like you do the live website (production)
• Verify your connection string is using the sa account locally. (if it isn't, change it)
• Backup your db and file directories at this time from your local running install (So you can start from here again later if need be, skipping steps above)
• Copy the upgrade package over top of your local install (ie. upgrade the site locally)
• Call it up in your local browser (of course)
  • Keep notes (I just copy/paste into a new outlook email) of any errors
    • I like to keep track of time here, I also tend to note if it seems something is taking long.
    • I also have my local task manager open to monitor cpu usage. 
    • I read any .log files for the SqlDataProvider version files (this should only happen if something fails, you see in dnn installer)
  • If you have any errors displayed from the upgrade process itself (remember, you haven't seen the upgraded site yet, you see something like 'click here to access portal') do NOT PROCEED yet. 
    • At this point, you need to determine why you are getting sql upgrade errorrs (or others) and fix that. 
    • if it is from an extension install (ie. a module/provider), this needs to be investigated, corrected too.
  • If you are error free, you should create a backup of your db at this time (keeping the previous one too for now) and name it something with _PostUpgrade in the title (so you can start from this point forward again in the steps, you may want the file system here too if installing modules). 
• I always view a few main pages on the site once upgrade is complete (even if i didn't install modules) just to see what is going on (and take notes if issues exist that were not expected).
  • Sometimes you get errors from things like a third party dll dependency shared by modules (like Telerik)
    • The module API really hasn't changed much between versions (for awhile now), Ajax/jQuery being the only exceptions.
• At this time I typically install modules (if you didn't do this at time of upgrade, I typically don't so i can go through these steps). 
  • If possible, I like to install modules and test one by one. (ie. go view a page with the module).
  • Investigate anything that isn't fixed by a module upgrade. 
    • Considering the cost of third party modules (not built custom in-house) I usually just upgrade to the latest version available for my new dnn version (I typically use latest released when upgrading core). 
• I consider it successful when no new errors exist in the DNN Event Log Viewer
  • Hopefully, there were no errors in the event log prior to upgrade (but I know this may be beyond your control at this point). 
• I recommend, at this point you try to make sure the DNN Event Log Viewer is clear of any new events. 

Now I know this may seem to be time consuming, however, if it is critical to minimize downtime I feel this is the best approach (ie. preparation). More often than not I find and fix my problems (if any occur) rather quickly this way too so I feel like it really doesn't take that long. Hopefully, this bullet list can help you (or someone else) in a future attempt. 

Your instructions were very helpful and the upgrade was a success overall. I still have a few modules not working but I think that the biggest secret was using SA for connectivity. Wonder why I haven't seen this tip anywhere else?

So now the test succeeded I can move on. thanks for your help!