I've finished Dawnguard and chose to cure Serana of vampirism, but now I think I'd like to have the option of become a vampire lord and getting bloodcursed arrows. I've done some basic console fiddling but was unable to faithfully reproduce a vampire lord transformation (adding the vampire lord spell turns you into a weaksauce version that dies in 2 hits on Master). Mods and console commands welcome, though I'd prefer something along the lines of Valerica taking on Serana's "duties" or simply turning Serana back into a vampire lord.
Late answer, but there are 2 ways to become a vampire lord:
- Accept Lord Harkon's gift during the Bloodline quest
- Receive it from Serana after Chasing Echoes quest (which in your case you can no longer do unless you reload a previous save)
Provided that you have turned down lord Harkon's gift, rejected the option to be turned into a vampire before entering soul carin and also during the quest "Kindered Judgement", and proceeded with curing Serana, there isn't any way to get vampire lord status other than experimenting with console command. Sanguinier Vampirism is always available though. PS Valerica won't even give you bloodcursed arrows let alone the gift of Vampire lord when you inform her of Harkon's demise.