I've tried to build my own letter following the CM - Building Guide,
the form guide also works fine, but if I change something in the Guide the FormGuide is not recompiled when I reopen it in workspace.
Is it possible to look up if the xdp or the guide has changed an force the client to load the current version?
Lc should be regenerating the FormGuide when it detects a change in the template's timestamp, or when the timestamps of any related collateral become newer than that of the form itself. Any FormGuide change must be applied, and then the form must be saved via the WorkBench Designer's save button.
Also, you can login into Lc adminui and navigate to the Forms admin page where you can manually clear the current forms cache.