Hi Benjamin,
I didn't verify this via code but I did play with ungroup, modification, regroup with Revit UI,.. however I didn't find parameters losing, do you mean the parameter values missing? If so, you can reserve the parameter values firstly and then assign the values after regroup via Parameter API, can this help?
Besides, in order to explore this matter further, we will need to provide a minimal reproducible case for the development team to analyse in depth.
In order to understand exactly what you mean and be able to research possible reasons for the discrepancy between the observed and expected behaviour, they require:
· A short exact description of what you are trying to achieve.
· The behavior you observe versus what you expect, and why this is a problem.
· A complete yet minimal Revit sample model to run a test in.
· A complete yet minimal macro embedded in the sample model or Visual Studio solution with add-in manifest that can be compiled, loaded, run and debugged with a single click to analyse its behavior live in the sample model.
· Detailed step-by-step instructions for reproducing the issue, e.g. which element to pick, what command to launch etc.
· http://thebuildingcoder.typepad.com/blog/about-the-author.html#1b