Somebody on the site said it was possible to transfer a GTA Online account to a different account if you call Rockstar support. Is this true?
While in theory it is likely very possible, given the customer service folks of game companies usually have a lot of power to administer and change user's accounts, Rockstar was very clear in the account setup screen that the account is not transferable and it can not be changed, so my official answer would be "No, it's not possible” (I just set up my GTA V/Online account yesterday).
That being said, nothing is keeping you from trying to contact their customer support and asking — the worst they'll say is no, and you would be no worse off than you are right now.
I have not seen any cases of a fully transferred account, but there have been occasional cases where someone's rank and cash equal to at least some of their current cash and the value of their bought properties have been applied to the new account.
Although most that have asked have been turned down and the ones that have succeeded often get a line similar to "This is not something we usually do but seeing your high involvement in the game we'll do this once for you".
So that it'll work is possible, but unlikely, although chances seem to go up the more you have played. It's possible having bought shark cards also contributes to the "high involvement".