Hello Michal,
you said to change "maintenance contract", If this is to migrate the systems from one Contract Maintenance from SAP Customer A to SAP Customer B, you can founf information on that SAP Note:
Note 1027439 - Migrating "secure storage" across customer numbers
Also you can send a OSS Support Incident to "BC-SEC" better than "XX-SER-LIKEY".
I did that sometimes, and the template that you need to use is something like that:
----
stem name: DEV
previous inst.name: 0099999999
previous customer name: CUSTOMER A
new instalattion number: 002027999
new customer name: 0000195999 CUSTOMER B
assertion system exist: yes
-------------------------------
system name: PRD
previous inst.name: 0088888888
previous customer name: CUSTOMER A
new instalattion number: 002027999
new customer name: 0000195999 CUSTOMER B
assertion system exist: yes
-------------------------------
Hope that can help you,
Regards,
Luis