Epic villain pools

At lvl 35 if you are a villain already you can simply do the arcs that unlock a patron power pool. From that point on anytime you respec you'll be able to choose any of the patron power pools or the standard epic power pools.

If you are NOT a villain however you'll have to do tip missions to change your alignment so you can go to the Rogue Isles and do the patron arc of your choice. Then you'll have to do a series of tips again to transition back to being a hero.

Once unlocked you'll always have access to the patron pools regardless of your alignment.

Just to clarify this point, it's not enough to have access to the Rogue Isles; you must be an evil alignment - Villain or Rogue. Vigilantes cannot speak to Arbiter Rein to start the Patron Arc.

It takes 10 Alignment tips and one Morality tip to make ONE alignment shift. Furthermore, Alignment tips are capped at 5 per day. If you find a way to do more than 5 (typically by joining a team), you still will not get credit for more than 5 at a time. The Morality tip is not affected by the cooldown period. This means that it takes at least 2 days to shift one alignment.

Assuming that you start as a Hero, you need to do 10 tips (selecting the Vigilante option) and one Vigilante Morality mission (pay close attention to this! If you fill both the Hero and Vigilante bars, either type can drop! If you do the wrong one, it empties BOTH bars, and you have to start over the next day).

Once you become a Vigilante, you need to go to the Rogue Isles. You can cross over via the shared zones: Pocket D, Rikti War Zone, and The Midnighter's Club. You can also use the submarine located in Indeppendence Port and Sharkhead. You can't use Ouroboros (it's technically two different zones, and you only have access to one at a time), nor can you use PVP Zones (Vigilantes are still Heroes, and Rogues are Villains. Approaching the other side's base will get you attacked).

If you stay in Hero Zones, only Hero/Vigilante tips will drop. You'll be able to switch between those two alignments, but not progress towards Villain. Once you're in a Villain Zone, Villain tips will start dropping. Complete 10 of those, plus a Villain Morality mission to become a Villain.

Once you're a Villain or Rogue, and level 35 or higher, you can speak to Arbiter Daos. If he doesn't show up on your contact list, go to Grandville, and walk up to the tower that's behind the giant Recluse statue. Arbiter Rein is just outside the door to the tower. He'll give you a mission to learn about the Patrons, and then will let you choose one. This is a permanent choice! Once you pick a Patron, you can NOT get access to the other three by ANY means! Fortunately, all that your Patron determines is which badge you get, and what missions you run - completing the first arc from any one of them unlocks the powers from all four. Once they're unlocked, they stay unlocked forever.

After you've become a Villain, you do more tips, this time selecting Rogue, to switch to that alignment. You can do this before, after, or during your unlocking of the Patron; as long as you're an evil alignment, you can work on any Villain contact.

Once you've unlocked a Patron and switched to Rogue, head back to a Hero zone and start doing Hero tips to switch back to a Hero alignment. DO NOT do this until you've unlocked your patron powers! You don't need any in your build; you just need the badge that unlocks them. When you go back to Hero, you'll lose access to your Villain contacts. If you haven't finished the arc, you'll be locked out of it until you go all the way around again.

Now that you're a Hero again, if you plan on earning Hero Merits, you'll need to re-affirm your Hero alignment. You do this by simply doing 10 more Hero tips and a Hero Morality mission when you're already a Hero. If you don't collect Hero Merits, this step is optional.

Total time (minimum values):
Hero > Vigilante: 2 days
Vigilante > Villian: 2 days
Complete Patron arc: A few hours (5-10 missions)
Villain > Rogue: 2 days
Rogue > Hero: 2 days
Reaffirm Hero (optional): 2 days
Total: 8-10 days, assuming maximum progress is made each day.

One additional thing of note: If you are already collecting Hero Merits, spend them before you start your trip! If a Hero switches to Vigilante or a Villain switches to Rogue, then all of your Hero/Villain Merits will be DELETED. They WILL NOT come back when you return to your original alignment; they will be lost forever.

Hopefully that answered all your questions about side-switching and Patron Unlocks. If not, feel free to ask, and I'll try to help more.