@Onats Ong Upgrading apps from pre Grails 3 will be a bit more complicated as a few things have moved like src/groovy moved to src/main/groovy. Also some bad practices are no longer supported, by default. have upgraded a large grails-app from 2.5 to 3.3(painful) to 4.0(not so bad) and my postmortem notes are here: https://dev.to/virtualdogbert/grails-2-5-5-to-grails-3-3-10-postmortem-520a https://dev.to/virtualdogbert/grails-3-3-10-to-4-0-2-postmortem-5kj I wouldn't suggest that you do incremental upgrades and go for either 5.3.4 or 6.1.x. I would go for the last 6.1 at this point as it's been out for a while now and will be supported longer.