Can I change "online" the securitykey and get a permission from one or another DB?
No, the security key is used as the seed for the byte[] that is the key used to encrypt/decrypt the us_Data field. So, if you change the key, suddenly, you won't be able to see anyones passwords and the users will all become invalid. Hence why the key exists within the SecurityMaintenance editor for adding users at design-time. The data must be encrypted using the same key to be decrypted at runtime.
If I handle the aftersave and duplicate from one DB to another EXACTLY the SFS_users row, not having problems with us_data?
Yes, you can copy a user from one database to another with no problem copying the us_Data field, IF (as I mentioned above) the MicroFour.StrataFrame.Security.SecurityBasics.SetSecurityKeyAndVectorForUserAuthentication() method was called with the same security key for all .EXEs that use the access the users.