It is a fairly painful process - I am using Codex and Claude as a first pass for converting things, but everything has to be checked manually and often fixed or rolled back to try again. Too often instead of "removing" piece of code that is not necessary anymore, AI would try to add a 100 line workaround for it because it would really rather add things than remove them.

It is, however still a faster process than doing manual rewrite. Smaller components are a breeze to convert from older Vue 2 to Vue 3 composition API.

Rewriting entry point app.ts with bunch of custom plugins was a horrendous experience that eventually was done manually. Move from Vuex to Pinia was also 50/50. Some things were easy, some things were not and had to be done manually.

Then there's a process of switching from unsupported packages which is a whole other can of worms.

Overall - if not for AI I'm not sure I'd have gone with an upgrade at this point. Yes it requires careful review and testing, but a ton of trivial stuff was done very quickly.