Need an idom or proverb

[sheheryarnoor]

Hi everyone
I don't know where to post to my request so i am just gonna post it here
"When someone gets money or seat and then he forgets his past"
In Pushtu it is"A cat was queen but she hadn't tasted milk after tasting milk she left her's Kingdom"

[sheheryarnoor]

Another Example is of Obama who promised that he will do that and that after getting presidency but did nothing so we can say that it is not his fault anyone who gets presidency forget everything so i need an idiom or proverb for this kind of situation