This is primarily a "Flavor" question (or maybe not given the long range ramifications) that came up during a game session from reading the late 4th edition "Dungeons and Dragons" comic where the Tiefling Warlock Tisha was clearly unhappy with her Pact.
Fast forward to 5th edition…
It's a pretty simple concept: Can Warlocks "find" new Patrons?
Paladins can become Blackguards, sure, but that's a matter of falling from grace and being accepted "Paladin level for Blackguard level".
We're playing 5th edition but we're more familiar with 3.5th edition rules so we could simply be missing something.
Note: We only have the Core books: PHB, DMG, MM. None of the others. 🙂
Best Answer
As per this question, there's no rule which allows a character to re-spec their class options. However, the Adventurers League rules allow for a character re-spec below level 5, and outside of that, the DM has the power to change or ignore rules to grant any player a re-spec, and a responsibility to make sure all the players are having a good time.
Lore-wise, a warlock has a responsibility to serve their master, and this implies that they can be fired, too:
This suggests that you can find yourself unable to progress after disobeying your patron, and it's entirely possible that you might catch the attention of another patron who offers similar or different powers. Apprentices can acquire new masters, and several clerics in D&D lore have changed deity.
It's certainly not broken or unbalanced to re-spec (unless you do so frequently so as to gain an unreasonable amount of versatility; once only should be fine). It's even easier if you happen to be happy with your existing powers, but just want to change the patron for fluff reasons, since fluff is entirely within the ability of the player or DM to decide, and the rules hold no sway here.