Name changing will kill minecraft. Friends won't know who their friends are... Trolls can change their name every 30 days and use VPN's to connect. Name changing is purely for server trolls and accomplishes nothing but confusion and frustration!! Name changes will also destroy forums. Nothing will match. Trolls can change their name, go on the forum with a nasty name and say nasty things... then do it over and over again - Every 30 days !! It will completely break Minecraft after countless changing to the point that nobody will want to play the game anymore. A lot has been in decline since name changing and Minecraft will never be the same. These days, everything is in UUID so looking up a player involves first converting the name to UUID and then hunting through the alien codes to try to locate their YML... No more sort by player, etc - It's all gone !! The name changing update is the thing that will destroy any server owners desire to continue (unless they only care about donations and not players) ~~ RIP Minecraft !!

If you ban/op/whitelist somebody they will stay like that

Well if I ever get on a server, my nickname will be related to my username, for example Stuff_Of_Evil instead of Evilstuff or something so other players can tell who I am.

It'll probably be more of a nickname type thing. Their username will remane the same at all times and they can chose a nickname that will show up in game. However, once you ban their nickname it also bans their username too. I don't know if this is how it will be done, but I assume it's a good bet.