Can A Warlock Change Patrons?

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. 🙂

How do you get Favors for use with Patrons presented in Tasha’s?

In Tasha’s, new Patron rules are presented and a number of the patron Perks require calling in, or using a favor. I may have missed it, but how does one gain a favor? There are a number of references to downtime activities, so is it the same as gaining a religious favor (one of the Patrons is religious organization)? Or, is this simply a DM call: A favor is gained by completing quests and the like on behalf of the Patron? That might raise a follow-on question in regards to perk and being your own Patron.

What named creatures or beings would fit thematically as patrons of a Dark Pact warlock?

What named creatures or beings would fit thematically as patrons of a Dark Pact warlock? By “named”, I mean a being that exists in some lore (primarily but not necessarily limited to Forgotten Realms lore).

The only example I have seen is in the Dragon Magazine #381, where it lists (on page 48) a creature called Yorgrix, Weaver of the Poison Web, which is a demonweb spider living in the Underdark.

Before almost an entire page about that patron, it more broadly references this (also on page 48):

Whether drow, dark spirit, or creatures of terror, dark patrons lurk in the hidden places of the world.
    Common Patrons: Demons (especially servants to Lolth), purple dragons, and legendary dark spirits of the Underdark such as Yorgrix

So possible named patrons would presumably include any known Demon Lord (or any named demon), and any named purple dragon, but “legendary dark spirits” is a bit more vague…

The sentence above that mentions “drow, dark spirit, or creature of terror”, so does that mean a drow mage would make an appropriate patron, kind of like a Sorcerer-King Pact (as I understand it)? This is more of a secondary question, though; my main question is about named beings that would work as patrons for a Dark Pact warlock.

Are there any other named examples throughout 4e source material? These “legendary dark spirits” are of primary interest to me, I’m not really interested in gathering lists of named demons or purple dragons (that sounds easier to simply Google, but expanding on what these “dark spirits” are is something I feel would benefit from 4e expertise; note that I only have knowledge of 5e).

Related: What books cover Star Pact Warlocks and their awful masters?
Related: What books cover Fey Pact Warlocks and their awesome masters?