hi 999baz,

it depends, doing this update you will lock a big chunk of data.

First of all you must test it, use your development DB and test with 10000 rows, 100000 rows, or yours 2500000 rows, and estimate the time needed to complete the task.

Now you know the amount of time needed; have you an adequate time frame to do it without disturb the normal operation?. If yes you can schedule your update; if not you should update your table using a cursor and committing your changes after some amount of rows updated.