Hacking Questions about downgrade

[rattleheadeth]

I have one 2ds in firmware 10.3U and I want to install CFW in order to play 3DS backups (.CIA).
I have menuhax and a custom theme, I have to remove them before downgrading?
What files I need to downgrade (I see a post day ago where show certain files cause bricks)?
Do I lost some data after downgrading(like eshop games, saves, etc)?

My english is not too good (almost bad), because I'm from Chile, and no, I don't use Google Translator. THANKS