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. 🙂