Can I subsequently “limit liability” and take over these contracts from my old company?
No. It is not possible to transfer old contracts from the sole proprietorship to the UG (limited liability company): The UG is not a so-called “legal successor” and does not enter into the existing contracts of a sole proprietorship.
Reason of the legal regulation: The contracting partner is not to be surprised straight by the liability protection of the UG. After all, he has concluded a contract with a natural person and not with a limited liability company.
Illustrated by an example: If this were possible, one could simply found a UG for credit contracts of a private person and would then be free of debt oneself.
Your personal liability is only effectively excluded for all legal claims (and also private insolvency) when the UG (limited liability company) has been entered in the commercial register.