Upgrading Magento from an earlier verson such as magento upgrade 1.7 – 1.9 can become a bit of a challenge depending on the ecosystem of templating plugins and extensions involved. Assuming your install is fairly basic and you haven’t got a tonne of extensions piled up onto the core than this worked for me as a charm. I would strongly suggest that you utilise version control and as the database will be updated and modified as you upgrade Magento 1.7 to Magento 1.9 then it would be a good idea to export the database periodically.
To started with upgrade via Magento 1.7 – 1.9 via the Magento Connect portal to the latest code. After installing this I received a blank screen. After refreshing the page, I could see that half the upgrades for Magento 1.7-1.9 had been carried out. So after updating the remaining core modules which where still highlighted all went well.
After all the green lights on the update, when navigating to the /admin area I was greeted by lovely white screen of death. For some reason the Magento 22.214.171.124 doesn’t seem to finish so you need to copy all files from the Magento 126.96.36.199 tarball on the website into:
Magento 188.8.131.52 missing files
After that is done, you should be able to access the admin and should work ok.
Note this is assuming a vanialla install of Magento with a theme and possibly some minor extensions.