Is it obligatory to utter the transitioning Takbir before actually transitioning in prayer, and would doing so lead to a discrepancy in congregational prayer?

The takbirat (utterances of "Allahu Akbar") for transitioning between prayer postures are prescribed to be simultaneous with the action, so one should not start before it or finish after it, because their place is between the two pillars (rukns). Nevertheless, if the worshipper begins the takbir before the transition or completes it after, their prayer is valid, and the followers (ma'mumun) must follow the imam. As for the imam, he should not be told to delay the takbir out of fear that the followers might precede him, for this is contrary to the Sunnah.

Ibn Qudamah said: "The beginning of his takbir should be with the beginning of his rising, and its end with its completion." And also: "It is recommended that the beginning of his takbir be with the beginning of raising his head from prostration, and its end upon his standing upright, so that the takbir encompasses the entire rukn in which it is prescribed." This applies to the rest of the takbirat, except for the takbir for the sitting of rest (jalsat al-istirahah), which ends when the sitting ends, then one rises without takbir.