I find it helps to think of Warlock patrons as, well, patrons. You know, wealthy nobles who would pay artists to fund their art. It’s not really a business relationship - the patron doesn’t technically have any say over what the artist makes, they’re just kind of donating to enable the artist to keep doing what they do, because the patron likes what the artist does. That said, there is an understanding that continued patronage is not guaranteed, if the patron no longer likes what the artist is doing with the money the patron gives them. There is an incentive for the artist to focus on the kind of art they know their patron likes, and if their patron ever offers a suggestion of something they’d like to see from the artist, there’s an incentive to give that request special consideration.
Now, consider a similar relationship, but instead of an artist it’s a warlock, and instead of money it’s magical powers. Rather than a warlock having a contract with specific terms they must fulfill, a patron donates power to a warlock because they expect the warlock will do things with that magic that will serve the patron’s interests. The warlock isn’t technically obligated to do anything specific, but knowing that their magic is a gift from their patron, they are incentivized to keep their patron happy with the arrangement. And if their patron ever makes a specific request of the warlock, the warlock should give it special consideration.
On the mechanical side of things, the warlock gets their abilities from leveling up, just like any other class does. Consider this as representing the warlock’s skill with the power their patron granted them increasing, much like an artist’s skill increases as they regularly practice their art. As long as the power keeps flowing, the warlock is free to operate more or less independently. But, if the patron doesn’t like what the warlock is doing, that power is not guaranteed to keep flowing. For example, a devil patron may love it when their warlock uses their power to kill demons, but be less keen on them using it to kill devils. An Archfey may be all too happy to supply a warlock with power in hopes of simply stirring up some shenanigans, but become displeased if that power is turned towards maintaining order within a rigidly structured society. A great old one’s motives may be entirely inscrutable to their warlock, though using it may slowly drive the warlock to madness.
salute that the patron can't do anything about as a design intent
(answer was yes, I let players change things up in the first few levels as they're still getting a feel for the character/game/party needs n' dynamic) 
