With the 321XLRs finally rolling into the schedule in a meaningful way, thought it would be helpful to clarify how PE upgrades are currently working. As far as I can tell, the official policy remains:
For flights within North America, AAdvantage® status members receive complimentary upgrades to Premium Economy at the gate if upgrade-eligible seats are available.(link)

For those who've flown domestic PE, is this the case in practice (i.e., no advance upgrades to PE)? Also, are gate agents proactively ensuring that PE cabins are filled at departure with eligible elites, or are seats going out empty? With the significant loss of MCE seats on these birds, I am hoping there will be a meaningful opportunity to be upgraded to PE when J is not available. Thanks!