Thanks @rgb. I've reproduced this using your project and fixed it for the next release.
The saved method declares the parameter as edit, but its body uses medit. The compiler therefore correctly reports that medit is undefined.
Changing the parameter name requests a rename, but that operation is refused because the project already contains a compilation error. The bug was that the Parameters field continued displaying medit even though the declaration still contained edit. That explains the missing preview, the lack of a modified indicator, and why saving didn't retain the displayed change.
Rejected or cancelled parameter renames now restore the displayed text and show the reason directly beneath the Parameters field. I've also added regression tests covering this behaviour, saving/reopening, and undo/redo.
For now, change the body back to dc[CL_EDIT] = edit. You can then rename the parameter to medit through the Parameters field and accept the preview, which updates its uses automatically.
Tracked as Restore parameter text and explain rejected renames, marked fixed for the next release.
