I wanted to play Dawnguard without spoiling myself at all, but it didn't turn out to be a good idea. Having to choose sides so early and without a chance to think about it became as a total surprise to me and now I'm sort of stuck. I don't know anything about either side.

So now the vampires intrigue me greatly for two reasons:
1) I think it would be impolite to decline Harkon's offer
2) The Dawnguard seem incredibly bland, having no other motive to slay vampires other than... slaying vampires.

So if I take Harkon's offer can I switch to Dawnguard if I regret it? I mostly mean by the time I go to Ft. Dawnguard next time? I don't know much about the Dawnguard and they might turn out to be interesting people afterall. I've heard that you can revert vampirism or turn into it even after choosing sides.

no, you cannot change sides, even if you cure yourself of vampirism.

No, you won't be able to to. Make your choice wisely.

I don't think the DG are bland. Most of them lost loved ones to Vampires.

Yes you can.But you must do it after you leave the castle and you can't do any Volkihar side quests.