Provided solution for merging usernames from different domains does not work!

[Faryad Kashef]

Dear Community, 

We are migrating our domain from "at.xxxxx.com" to "xxxxx.com" (both in the same forest) and want our users to use their new usernames in the new domain while keeping the same permissions and contents from their old usernames. Apparently the only solution or workaround is this:

How to merge two users' content ownership and permissions in Confluence | Confluence | Atlassian Documentation

I did proceed with this and converted queries to MSSQL but at some point it returns errors. For instance:

ALTER TABLE user_mapping DROP PRIMARY KEY;

Returns an error like: 

Msg 3725, Level 16, State 0, Line 1
The constraint 'PK__user_map__E1CC8CC0D5F8' is being referenced by table 'LABEL', foreign key constraint 'FK_LABEL_OWNER'.
Msg 3727, Level 16, State 0, Line 1

I manually removed the key using the Object Explorer interface but I am not sure if it is correct. Again, while performing the next queries I get some errors with the queries. Has anyone recent experience with this?