To add more detail i think the specific change in question is that. A player will not enter contract negotiations to join a team he is not eligible to play for on the date the transfer is to be completed.
Previously he would enter negotiations but the transfer would take place at the earliest possible date that the player would be eligible to play. So for example under the old system, a european team could agree a fee and contract for endrick and the transfer would complete at that later date(usually the first transfer window after he turns 18)