If you change to another degree program, you can continue to use your existing PIN and TAN. This also applies if you take up a master's program at the University of Bremen after completing a bachelor's degree.
It is not necessary for you to re-obtain these documents, even though you might receive such a message as part of the general information sent to students.
If you wish to change your university, you cannot be enrolled as a first-semester student: You can only be enrolled in continuation of the semesters studied at the other institution, e.g. 2nd, 3rd, 4th semester. Admission as an advanced student is only possible if it accords with the program offered, i.e. advanced students can only be enrolled if the bachelor program is offered in the corresponding semester.
Transfer students are students from the University of Bremen or other universities who would like to change their subject and / or degree program. A change of subject usually takes place in the 1st semester of the winter semester.
Newcomers are students from the University of Bremen or other universities who are applying for a higher semester in a related field of study. The prerequisites for this are recognized credits of at least 1 semester from their previous studies.
Here you can find further informations.