The Union of European Football Associations announced drastic rule changes to European competitions, which will take effect next season.
Under the new guidelines, a player will be allowed to feature for two different clubs in the same European competition.
Also, a fourth substitute will be allowed in knockout ties that go into extra time.
As it stands, players signed in January are not permitted to play for their new clubs in the Champions League or Europa League if they had already featured in that competition.
For instance, Arsenal striker Pierre-Emerick Aubameyang is barred from playing for the London side in their Europa League campaign, and Philippe Countinho is also barred from featuring for Barcelona in the Champions League.
In the new guidelines, UEFA will also allow teams that have won the Europa League three consecutive times or at least five times to wear a special multiple-winner badge on the shirt sleeve.
Another change will see teams provide a list of 23 players, rather than usual 18, for the match sheet in the UEFA Champions League final, UEFA Europa League final and UEFA Super Cup.