Thanks Folks!

For all your help.
I had my Meeting about this Topic on Friday, and for now the "Problem" was discussed and because of my Tests the database Migration is postponed .

To tell my story with the problems I had, I can share some more Information:
- database table with two columns (GUID, VARBINARY(MAX)),
- development wrote database migration steps
- one checked for existing NULL or DATALENGTH = 0, and deleted those entries, if there were some
next step redefined the column (from NULL to NOR NULL), which increased (recovery model FULL) the database about the size of exactly that table, and the transaction log increased also about this amount
- I had to do a Manual shrink afterwards, but on a database where this table is about 800 GB or more, it lead to a expected "Migration" of many hours.
- so we skipped this for that Moment, and left the existing CONSTRAINT (checkung for NULL or DATALENGTH = 0)

Thanks all for sharing your knowledge/experience and hints.

C.-A.