I might add that in my view the migration issues seldom come from bugs in Archicad. Even if a new version adds new features that might have bugs, these shouldn't matter because old projects didn't use them.
The real issue is the library parts. Many of the standard parts, even if they look alike, have minor revisions. Some are of course bug corrections, others might be additions to handle the new program's new features. Some of those additions might create not anticipated problems in your old project.
Even library parts that have not changed at all might cause issues, because the new program interpretes them diffferently - it could be that it interferes with some new feature, or simply that the new program is sensitive to a bug in the library part that didn't matter to the old version. In the simplest case this bug sin itself might be harmless, but the new AC version's better consistency checking refuses to load it. This is bad enough - imagine a complete roof structure, modeled with customized profiled beams, that won't open in the new version because of some glitch in line xxxx of macro yyy etc. When you've closed this OK/Cancel dialog a hundred times or so, and you realize you can't replace the parts with new because they all had different customized dimensions, you swear never again!
This is why you should not migrate a project that's close to finish, unless there is something in the new program version that you really, really need. And even then, proceed with the utmost caution. Keep a complete untouched installation of the old program version with libraries n all on a separate hard disk, safely tucked away in a closet! (mac tip: SuperDuper!)
And this is also why I fancy Matthew Lohden's idea of the Unified File Format for Archicad: When you place a library object in the project, that part would completely (including all used macros etc) be stored in the project file, and would never be exchanged for another variety, regardless of which or how many libraries you had loaded at any given time, unless I explicitly tell Archicad to change it.
This idea would significantly lessen the headaches caused by sloppy library management, something I'm all too familiar with, being the primary culprit!
Thanks for reading this rant all the way!
AC4.1-AC26SWE; MacOS13.5.1; MP5,1+MBP16,1