A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
A player, 22 February 2012
to clarify
if a province has 2 dynasty cards in it (because its maximum was raised beyond one by a card like Plains of Otosan Uchi)
And then its maximum held cards is reduced (such as by the destruction of the Plains of Otosan Uchi)
I don't see any rule that says that should the maximum be reduced, excess of the maximum are discarded. I feel this makes it like attaching spells or additional weapons to shugenja and kensai respectively, and then losing aforementioned keyword, however a player I respect for his understanding of the rules felt if in a tournament he would call one of them be discarded and then ask the ruling on the forums. Figured I'd go ahead and check the ruling ahead of time (might encourage me to run it)
http://rules.l5r.com/CE_Areas_of_the_game#Provinces_.28out_of_play.29
Ben Higgins (rules official), 24 February 2012
to clarify
if a province has 2 dynasty cards in it (because its maximum was raised beyond one by a card like Plains of Otosan Uchi)
And then its maximum held cards is reduced (such as by the destruction of the Plains of Otosan Uchi)
I don't see any rule that says that should the maximum be reduced, excess of the maximum are discarded. I feel this makes it like attaching spells or additional weapons to shugenja and kensai respectively, and then losing aforementioned keyword, however a player I respect for his understanding of the rules felt if in a tournament he would call one of them be discarded and then ask the ruling on the forums. Figured I'd go ahead and check the ruling ahead of time (might encourage me to run it)
http://rules.l5r.com/CE_Areas_of_the_game#Provinces_.28out_of_play.29
You are correct, after Region is destroyed or discarded the "extra" card would remain, until it was brought in to play or was discarded. You would then follow the procedure for refilling the province as mentioned in the link to the CR that you posted. The province would then have a maximum of one space for a facedown Dynasty card, so would not have a empty spot to fill.
Note: Previous printings of Plains of Otosan Uchi did have an effect that would force discarding the extra card in the province when the Region left play. However, this was removed with the MRP in Emperor Edition. Please be aware of this if you are playing older versions of the card in tournaments.