Why did the First Mexican Empire replace 60% of its Regency?

Question

From September of 1821 until Agustín I ascended the throne, a five-man Imperial Regency governed independent Mexico. Initially it included Agustín Iturbide (the same), José Isidro Yáñez, Manuel Bárcena, Manuel Velázquez, and Juan O'Donojú, who dying soon thereafter, was replaced by bishop José Antonio Pérez. In April of 1822, the last three regency seats were reassigned to Conde de Heras, Nicolás Bravo, and Miguel Valentín. (This is due to The Mexican Empire of Iturbide by Timothy Anna, pp. 37, 56.) The high clergy thus went from two to zero seats on the regency, though Iturbide was very pro-clergy.

Why, after six months, did the composition of the regency change so dramatically?

Answer 1

Your question seems to imply that the Regency membership was under the influence or control of Iturbide, but according to Bancroft, at this later time these seats were reorganized by the Congress, who were in ongoing conflict with Iturbide, essentially attempting to stack it against Iturbide. From Bancroft, History of Mexico 1804-1822, pg 769 (emphasis mine).

As for the congress its hostility toward him was now openly expressed. By decree of the 11th of April the regency was reorganized the bishop of Puebla Manuel de la Bárcena and Velazquez de Leon who were too evidently subservient to Iturbide being removed from their positions and their places supplied by the conde de Casa de Heras Soto, Doctor Miguel Valentin and Nicolás Bravo in whom the congress placed the utmost confidence. Yañez was retained in his seat for the reason of Iturbide's known enmity to him.

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Wikipedia has a paragraph concerning this conflict between Iturbide and the congress for a little more background to the situation:

In the meantime, the governing junta that Iturbide headed convened a constituent congress to set up the new government. The new government had indirect representation, based on the Cadiz model, but the Plan of Iguala and the Treaty of Córdoba were clear that the order of things would be kept as it had been before the Cadiz Constitution. Thus, Iturbide and the junta declared that they would not be bound by the Cadiz Constitution but kept the Congress that was convened.[12] That led to division, which came to a head in February 1822. In its inauguration, Congress swore that it would never abide for all of the powers of the state to fall into the hands of a single person or entity. It, however, proceeded to assign sovereignty to itself, rather than to the people, and proclaimed that it held all three powers of the State. It also considered lowering military pay and decreasing the size of the army. Those moves threatened to reduce Iturbide's influence in current and future governments.

Iturbide assumed the role of Emperor a month later, and dissolved this congress in October:

Iturbide closed down the Congress on 31 October 1822 and created a new junta, the National Institutional Junta, to legislate in its place two days later, answering only to himself.