How often can I change my mind about vampirism or lycanthropy?
I'm not sure whether I want to become a vampire or a werewolf, or just remain a normal human being. I'd like a chance to explore all my options.
I know there are ways to cure vampirism and lycanthropy. And I'm pretty sure there's ways, at least with vampirism, to re-acquire the conditions.
What I want to know is if there is ever a point of no return? Is there a point past which you become immune to either or both of these, or a point past which one or both can no longer be cured? How many times can I change my mind before reaching one of these points?
Solution 1:
DLC Dawnguard Information
If you have the DLC Dawnguard you can flip back and forth between the too. From the Skyrim Wiki:
"In Dawnguard, if you are cured of Lycanthropy or if you become a vampire lord you can talk to Aela and she can make you a Werewolf again and it cancels out your vampirism vice versa. If you would like to be a Vampire Lord again, speak to Serana."
Skyrim without Dawnguard
Without the DLC once you are cured of Lycanthropy. You can not get it again, with the exception Hircine's Ring. As long as you don't have Lycanthropy, you can contract Sanguinare Vampiris (which turns you into a vampire) from a vampire. There is a 10% chance to contract it when they cast Vampric Drain on you. If you cure it you can contract it again with the same method.
The important thing to note is Lycanthropy cures Sanguinare Vampiris. So if you have Sanguinare Vampiris and then get Lycanthropy you will cease being a vampire.
Solution 2:
Lycanthropy limitation
With or without the Dawnguard DLC, you can't (without the use of PC-only console commands) switch back and forth as many times as you would like between being a werewolf and being a vampire.
According to the UESP wiki's "Aela the Huntress" article (emphasis mine):
If you have cured yourself of lycanthropy, there is no way to regain it in the base game. However, the Dawnguard add-on provides a one-time opportunity to restore your lycanthropy by speaking to Aela.
After being cured of lycanthropy, simply find Aela and speak with her. You will have the option to tell her you wish to regain the gift of beast blood. She will respond, "Do you, now? Hircine doesn't usually bestow his favor on the fickle. But you've proven your worth. I'm willing to try. Are you ready now?" If you tell her you need more time to prepare yourself, she will end the conversation by answering, "Go. Reconsider your plight." If you then return to her you can go through the conversation again, and it will continue as before. When you choose to tell her you are ready, she simply says "Hold still, then." Your screen will turn black for a second, then a magical aura will circle around you and a message will appear on the top left of your screen stating "Beast Blood Added". The conversation is now over, and if you choose to cure your lycanthropy again, there will not be another chance for Aela to restore it.
TLDR: Without the Dawnguard DLC, you can only become a werewolf once. With the Dawnguard DLC, you can become a werewolf twice, if you've cured yourself of lycanthropy the first time around.
Console commands
If you are playing on the PC and have the Dawnguard DLC installed, you can use console commands that will allow Aela to turn you into a werewolf after your second time to be turned (allowing you to easily switch to being a vampire and werewolf back and forth as many times as you would like). You can do this by entering the console commands then conversing with Aela to turn you into a werewolf, and conversing with Serana to turn you into a Vampire Lord (or just be a vanilla vampire).
You can enable the conversation option "I wish to regain the gift of beast blood" to Aela with the console command: resetquest 0200f899
If you encounter issues with the above console command, enter player.setstage F6090 100 to force a cure before using the console command to enable the conversation option.
Console commands info from: The Nexus forums post, "Dawnguard Regifting Lycanthropy "
Notes
Serana won't be able to turn you into a Vampire Lord if she has cured herself of vampirism.
Solution 3:
According to the wiki, Aela will only give you Lycanthrope once more if you cure it (with Dawnguard DLC), so there is a point of no return. You can only become a lycanthrope twice with the DLC, or once without it.