We have to distinguish the LRZ ID from the TUM ID.
Your existing LRZ ID like "t1234an" continue to be valid for the time being.
These LRZ IDs - if you have one - have been issued by your LRZ master user. Besides him nobody knows that you use this ID. Therefore a change of password for this ID does not affect your central TUM account.
In the medium term it is planned to replace LRZ IDs issued by master users by the TUM IDs issued by TUMonline.