I'm not sure about from 4.7.x to 5.x, but definitely going to 6.x you'll need to make enough changes that starting over and re-applying your markup / css would be the route I'd go. Any major upgrade like this warrants looking at nearly starting from scratch since there are always a number of new features that would now come out of the box that you might have done workarounds originally.
The theming guide (in Documentation) has information on updating themes. But the information presented only covers going from one major version to the next. You'd have to figure out the differences if you are skipping a version (say from 4.7 to 6.x).
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You'll Need Make Serious Updates
I'm not sure about from 4.7.x to 5.x, but definitely going to 6.x you'll need to make enough changes that starting over and re-applying your markup / css would be the route I'd go. Any major upgrade like this warrants looking at nearly starting from scratch since there are always a number of new features that would now come out of the box that you might have done workarounds originally.
The theming guide (in
The theming guide (in Documentation) has information on updating themes. But the information presented only covers going from one major version to the next. You'd have to figure out the differences if you are skipping a version (say from 4.7 to 6.x).