Is is now possible to migrate only the customer DLUs with a MIG-CUCM License and to buy the other Enhanced licenses as new ones?
The MIG SKUs are only necessary if the customer lacks current ESW/UCSS coverage. Assuming that's the case, yes. Just set the quantities appropriate to how many they have. Add a second line on the order and specify the quantity of UCL Enhanced licenses.
Would there a possibility to delete the suppliers DLUs from the system before upgrade?
Yes. You would need to review the LIC files installed (System > Licensing > License File Upload) and determine which license filenames add up to the number of DLUs that aren't theirs. You can then delete those license files through the CLI. file delete license
Or may I purchase a MIG-CUCM license (former 9x3 promotion) for the whole ouput from the UCT tool (in this example 1000xEnhanced User), even when there are not enough DLUs?
MIG SKUs don't generate new licenses, they only re-add the appropriate contract entitlements. Don't buy a MIG SKU for something they don't have.
As a footnote, I would work through this situation with the customer's Cisco AM to be sure everything is above board here. Only HCS allows licenses to be owned by a partner and loaned out to hosted customers so this sounds suspect.
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