DSDs are rare. DSDs that could cause sex to be wrongly identified at birth are very rare. Cases in which sex actually is misidentified at birth, and the mistake not spotted well before puberty, are astoundingly rare -especially within a modern medical system in a country not known for genetic clustering of these conditions.
In the incredibly unlikely event of it happening to a Brownie, the question of whether they might need to move up to the Scouts instead of the Guides is probably quite low on their list of concerns. I would hope the Guides would deal with it with tact and sensitivity, but it's not a common enough eventuality to need a dedicated policy, any more than they need one for a meteorite hitting the roof of the Guide hut.