I suggest you install x64 build as standalone version
this way you can have clean install and separate profile folders (in standalone install, profile folder is in installation folder where you installed Opera)
otherwise, x64 and x86 builds share profile folder
on windows, without changing anything, that is C:\Users\USER_NAME\AppData\Roaming\Opera Software
The main reason for an update is for security flaws to be patched, keeping the browser as safe as possible.
Unfortunately, other things are often introduced as well as security fixes, which include changes to the way the user interface looks and works, and the changing or removing of facilities, which some users may have asked for, but other users may not like.
The problem with forced updates is that if you don't like a change that has been made, you have to jump through hoops to roll back to an earlier version and prevent it from updating itself again!
You are also left potentially with an unsafe version too of course.....