Is there a legal ruling (hukm shari) for those who missed part of the prayer (masbuqeen) to move from their place to complete their prayer after the Imam's salutation, or is it merely a custom?

It is disliked for latecomers (maṣbūqīn) to change their places of prayer after the Imam has finished his prayer, if there is no need for it. However, it does not invalidate the prayer unless it is repeated frequently or involves a long distance, such as about five rows.