Okay, I see your point but I wonder if Charles would actually be willing to let his brother be the heir of Castile and Aragon over him.
Furthermore that still leaves an important question: who does Anna of Bohemia and Hungary marry if both Charles and Ferdinand are taken?
Never mind you just answered my second question.
Well if grandad Ferdinand is able to persuade the Cortes of Castile to the switch between brothers (and if Catalina is regent there is pretty likely he would be able to overcome their mistrust of Ferdinand‘s plan, unlike OTL) then Charles has no other choice than accept the situation... Pretty likely who the two brothers ATL will have a very distant relationship, being at the best allies of convenience against a France who is a thorn in the side for both of them.
Remember who in theory Juana is Queen of Castile and heiress of Aragon but she is locked in a monastery under the pretense she is mad.
Castile surely was not thrilled by the idea of being ruled by another Burgundian, after luck had freed them from his father, when they had a nice, native alternative ready AND accepting this exchange is not equivalent to leave full power in Castile to Ferdinand as Catalina is both the regent in Castile AND support this exchange.
Maximilian will need a lot of patience for persuading Karl to stay calm and not worsening the situation while cursing Ferdinand but at the end he can not blame the other King who has just made his own interests