Only thing limited by your pact are your two At-Wills - every warlock gets Eldritch Blast, and their other At-Will is determined by their Pact.
Humans can get an At-Will from another pact, and I think there is a feat from Dragon that can get you another At-Will, but that's about it.
And there's nothing that says you can't retrain one of your at-wills at a later level.
And there's nothing that says you can't retrain one of your at-wills at a later level.