This is one of those cases where you really need to hire a specialist, rather than listen to the internet. Sure, you can't port your company out of germany, but there is nothing stopping you re-structuring so that you have an umbrella company based in the country of choice.
Agree about a specialist, but probably moving an existing company into an umbrella structure is likely to be considered similarly to a sale for tax purposes. In the UK, you have to specifically request HMRC to authorise any such plans if you want to avoid paying tax on it, and even then I believe it's only (relatively) simple if you're doing a straight share swap - so the existing company becomes fully owned by a new…
However that was UK/US and some EU but non-german subsidiaries.
I suspect you are right about movement being considered a sale. My assertion was imprecise as I suspect it requires an upfront company layout, rather than post-hock.