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.