The website was working just fine before the upgrade process, but time for a final back-up before pressing the upgrade button. Now let's check if every thing working fine. This list applies to most widely used content management system core upgrades and the more problematic issue of php versions. The recent switch to php7 did kill off a few of the older plugins, modules and components, but is definitely best to get-rid of obsolete unsupported code. There is always an alternative from the open-source community - I've just found adminexile to meet a new need.
Basic routine is :
Switch on the error reporting to development and see what we get.
Send a message through the site contact form - need to know that's working just fine. Of course check with the site owner that the message was received.
If you have any suggestion to add to this list please leave a comment.