I see many late-joiners in the congregational prayer changing their places after the Imaam finishes the prayer. They may move forward or backward in order to finish their prayers instead of remaining in the same row in which they were praying. Is there is a religious ruling in this regard or is it a mere habit? May Allaah The Almighty Reward you generously.

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

If late-joiners change their places in the congregational prayer after the Imaam finishes the prayer without a need for it, this act is against the Sunnah of the Prophet, sallallaahu ‘alayhi wa sallam. However, it does not invalidate the prayer unless it reoccurs or the distance is significantly far (i.e. about five rows). 

Allaah Knows best.