Well, I starting trying this upgrade months ago, but never had the time to get the bugs ironed out - so the last week I decided that now was the time to deal with it. Man have I wasted a few days on this!
Note, everything here is done on XAMPP localhost environment (OSX). This environment appears to be working properly.
What I've done - starting with a fully functional 2.0.25 site, my main add on was shipping - Canada Post - which added a few items to the database, which seem to fit in with 2.5.x version too, namely product_length, _width, _height, plus a couple of other fields that only Canada Post module worries about.
So thinking that is not going to be a problem I run the upgrade SQL function to bring the 2.0.25 database up to 2.5.0. That works a treat - database goes from 101 fields to 126 fields (two admin spots - one called admin, the other called administrators), and the root OSCMax 'store' works just fine with the updated to 2.5.0 database. Problem is the /admin does not. I get only the top half of the screen - down to the bottom of the menu bar, and a number of sub-menu items are missing. Missing include ALL of the modules sub-menus, Configuration just shows "Google Maps", and Administration is missing a few things in the middle including Site Maintenance.
Now if I CHANGE the database from the modified one, to a database that is 2.5.0, but empty then /admin seems to run just fine...all the drop downs are present, and the screen is full of info (as expected and seen on basic 2.5.0 installations).
So, I'm thinking the problem is more related to either the order of the items in the database or something is missing/present that is being misread by the 2.5.0 admin. Probably another add-on...hmmm...well, when the admin screen was broken it did have 'Security' on the far right, and glancing at that tabs' sub-menus indicated a problem...so I then decided to delete Sitemonitor references from the database and found it hiding in "sessions"! So, I simply deleted all the 'session' data (leaving structure alone of course) and now /admin works much better. Still am missing a number of drop down menus - as above - but I now get a proper screen.
Obviously there is something else in the data bus I have to extract to get it running.
So, if people are having trouble with upgrades, start with your database 'sessions' - back it up, then try deleting the data and see if you now get better operation.
Work in progress...
Images of results so far:
Administration Drop-Down Menu
Catalog Drop-Down Menu
Modules Drop-Down Menu
Configuration Drop-Down Menu
Customers Drop-Down Menu
You need to click on those links because I couldn't upload these pictures as this OSCMax site complained they weren't valid image files!
More problems with database are showing up...undefined variables, etc. lots more work ahead...