Is it permissible for the website to change the conditions for receiving the prize after it has been won and to demand a gift of lesser value, and is it obligatory to inform members of the website's actions that involve lying and fraud to attract participants?

If the competition is in Islamic sciences or beneficial knowledge, and the prize is offered by the forum administrators, and the competitor does not pay money for participation, then there is no harm in it. The prize must be given to the winner, and it is not permissible to retract that. If this is not done, then there is no harm in condemning whoever does not fulfill their promises.