All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, is His slave and Messenger.
Lifting the pants up to the level of one’s thigh deliberately or out of forgetfulness does not nullify the ablution. There are specific and known nullifiers of ablution listed in the books of scholars, and they do not include what you asked about. For more benefit on nullifiers of ablution, please, refer to Fatwa 84093.
As for the second question, each of the five daily (obligatory) prayers has its specific time, and it is invalid to perform the respective prayer before its prescribed time, and it would be insufficient in terms of clearing a person’s liability from the obligation in this regard. Allah, The Exalted, Says (what means): {Indeed, prayer has been decreed upon the believers a decree of specified times.} [Quran 4:103]
Ar-Rawdh Al-Murbi‘ ma‘a Haashiyatih reads: “Each prayer has its specific time prescribed by Allah, The Exalted, and if it is performed before such time, it is invalid. The Muslims unanimously agreed that the five daily prayers have specific and prescribed times, and it is invalid to perform them before such times, and it would be inadequate in terms of clearing one’s liability from the obligation in this regard.” [End of quote]
However, the time of the prayer is not conditioned on calling the Athaan (call to prayer); if a person is certain that the time of the respective prayer has started, the prayer would be valid and sufficient in terms of clearing his liability from the obligation in this regard.
Allah Knows best.