I'd say the former. But interesting question. Also, is it possible to remove a mowhoo tattoo and get your old sorcery back?

I think that's up to the GM to decide, but I'd give the player their lost points in Mowhoo so they're still epic, and if they lost the tattoo (arm cut off, etc.), then who knows, maybe their old magic comes roaring back! :) Maybe it's all sucked away, maybe you have to go on a quest to get it back, but with awesome bonuses when you do. :)

 

I'm thinking that if a Sorcerer Hero opts to get a Mohwoo tattoo, then they're pretty much "starting over" from scratch and that any previous Sorcery Advantage Ranks are lost.  So if the player invested heaviliy in something like Hexenwerk or Glamour, then I'd suggest making sure it's very clear to the player that if they take a Mohwoo tattoo and want the mechanical effects of said tattoo, they're going to loose their previouis toys in favor of the "new shiny."  Of course, they could just get a tattoo that mechanically doesn't do anything, but perhaps narratively might instead be invoked for a bit of Flair with certain Risks.

It's been a long-running theme of 7th Sea that generally speaking, different types of Sorcery tend not to mix.  Yeah, you had the Half-Bloods in 1e that could pick the Sorcery of two separate Nations, but those Half-Bloods were generally stuck with the lower-end spectrum of abilities that pureblood Sorcerers could eventually move beyond and get to the really juicy powers, so being able to use two types of magic carried a notable cost beyond the heavy investment of XP required.