How to anonymize user names in a data set

I did this a long time ago and unfortunately I cannot remember how I did it.

I have a large dataset of over 90 000 rows. In each row, there are long texts. And in these texts, there are usernames, identified with the symbol @ before the name. What I need to do is to anonymise these user names. I remember that I was able to substitute them, from @ username to @ xxxxxxxe. Is this still possible in excel without making a lookup table? There are thousands of usernames, it would be very difficult to make one.