UPDATE SomeTable

SET SomeColumn = 'domain2\John'

WHERE SomeColumn = 'domain1\John';

If I am still misunderstanding you, then please post your question in a more precise form. No "table something like this" but an actual CREATE TABLE statement. No "data something like this" but a few INSERT statements with sample data, carefully chosen to illustrate what needs to be done. And a list of the exact expected results based on that sample input, plus an explanation to clarify.