As @StingyJack mentioned, I tried to redefine the branch, creating a link with unreasonable merge, but without actually updating any files in the child branch during the unreasonable merge .
Using the TF merge command with the /discard allows us to create relations between branches without changing the child branch and disconnecting relations without pending merges between them:
tf merge $/Path/To/NewParent $/Path/To/Child /baseless /discard /recursive
Then you can simply override the child branch for the new parent branch.
(At first I tried to merge the spaces or change the comments and ignore the rest of the unreasonable merge changes, but this did not create a relationship to enable the redefinition (and canceling 70,000 changes took 45 minutes + each time)).
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