Here's my postmortem of what I did in an upgrade from Grails 2.5.5 to Grails 3.3.10 I figured this may help someone... The app I was working on was...
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Because you wrote:
Was left without QA for over a month
Does it mean that the upgrade "just" took 5 weeks for one developer?
No it's just one of the things that slowed things down, because it took longer to find where things broke. Because of the way the app was being developed the upgrade was dragging over months, instead of doing a cut over, focusing on making it work, and doing a smaller release, which would've been faster.
Very useful, thanks!