tldr: It's ok, no possible data corruption. 

Asked at the mailing list too, and they explained that the subvol UUID 
is just used a sanity check for `btrfs send` and `btrfs receive`. 

> ... 
> The UUIDs on subvols are only
> really used internally to that filesystem, so the kernel doesn't have
> a chance to get confused. The main thing that could be confused is
> send/receive, but that's a matter of possibly losing some validation
> (thus allowing you to do something that will fail) rather than causing
> active damage, as in the duplicate-FS-UUID case. 
> ... 

from http://thread.gmane.org/gmane.comp.file-systems.btrfs/50909/focus=50917 

Now I can sleep better :p