Is it correct to give out 2.5% of the money each year on August 1st for money that has reached its hawl (one year of possession) and money that has not? Is it permissible this year to limit giving Zakat only for money that has reached its hawl, or must one continue as before? Is it permissible to change the Zakat payment date to Ramadan 20th each year, and how is that done?

For zakat to be obligatory, it is a condition that the nisab (minimum threshold) is reached and a hawl (full lunar year) passes, calculated by the Hijri calendar. It is not permissible to expedite zakat before owning the nisab, by scholarly consensus. However, scholars differed on the permissibility of expediting it before the completion of the hawl. The Malikis hold that it is not permissible to expedite it by a long period, unless the hawl is nearing completion, such as within about a month, even then with karaha (disapprobation). Most scholars, however, went to the opinion that it is permissible to expedite it before the completion of the hawl without karaha, especially if there is a benefit, such as the need of the poor. They cite as evidence the hadith of Al-Abbas, whom the Prophet (peace be upon him) permitted to expedite his zakat. The more preponderant opinion is the permissibility of expediting zakat before the completion of the hawl if the cause for obligation exists by owning a complete nisab. As for the money you acquired during the hawl, if it resulted from the initial amount, then the hawl of the profit follows the hawl of its origin. If neither of them resulted from the other, then you have the choice between combining the later amount with the earlier and making one hawl for both, or making a separate hawl for each amount. It is not permissible to delay giving out zakat, but it is permissible to expedite it. One must pay attention to the difference between the Hijri and Gregorian years when calculating the hawl of zakat.