what cao pi did was to hit the final nail into cao cao bad reputation. pls ignore the novel storyline, the real history cao cao died as a court official of the han dynasty. As for the real reason why cao cao didnt force han xiandi to abidicate will never know as only cao cao himself knew.
as for guan yunchang, his reputation was over hype. the battles he won by himself can be count with one hand. let see, the two battles he fought for cao cao against yan liang and wen chou and also capture of changsha with 300men, that it no more i can remember. even zhang fei won more battles alone than guan yu.
he also a petty man, he very unhappy when he receive edict from liu bei that he was confer as one of the "five tiger generals" together with zhang fei, zhao yun, ma chao and huang zhong. He can accept being same level as zhang fei and zhao yun because they together for many years and ma chao are of noble birth but he tulan huang zhong being a captured enemy being same rank as him. it only a humbe letter from zhuge that evalvated guan to head of five tiger generals that he willing to accept the conferment.
from Liu Bei's POV, he damm koh-lian...got no talents to use...although amongst the 3 leaders,he is supposedly the most popular (post history) but in order to retain loyalty he has no choice but to elevate all 5 of them to "generals" . Cao Cao has the most talents. Liu Bei worst.
Guan Yu was revered more of his loyalty - which was underlined during time when he surrendered to Cao Cao under conditions.