Is there any yaoi manga where everything is as usual, uke is cute, seme serious and hot, but then AFTER seme and uke already had sex, uke wants to try to be seme? Or seme wants to be uke? And then the switch roles? Like the time in Love Stage!! Where the sweet uke almost "did" seme. So... I don't think on manga where guy who looks like uke is actually seme...

First thing that always comes to mind with a 'uke turned seme' is Three Wolves Mountain. I'll post more after I've looked through some of my bookmarks, because there's definitely more out there that I can't name off the top of my head.

You may want to try looking through the reversible couple tag in the mean time: http://https://www.mangaupdates.com/series.html?category=Re versible+Couple

That happens in Himananode Hajimete Mimashita. It's by Sakyou Aya, the one who wrote Kuroneko Kareshi no Itoshikata.