here at objektfabrik we've made quite some migrations from older VisualAge versions to Version 7.x. Most of the problems we've encountered had very little to do with changes made to the product by IBM or Instantiations. Most problems are caused by the quality of the code management (meaning configuration management and load structures) in the projects themselves.
We hardly ever needed more than 10-15 person days to do migrations - even from old versions like 4.5 to recent product releases. A side effect of a migration can be that you learn a lot about what you can do better in managing your code. And, of course, you benefit from new features.
I gave a presentation on migration at the VA Smalltalk Forum 2007, the slides are available for download at: http://www.objektfabrik.de/Downloads/VA ... Tuchel.pdf
(The slides are in German).
An english summary of the talk can be found at: http://www.cincomsmalltalk.com/files/ja ... eports.pdf
(Thanks to Niall Ross).
And, of course, feel free to contact me in case you are looking for help with migrating