If the high /21 of your /19 is not Allocated, you just assign it to them
and add ONE announcement of the /21 to supersede with the current /19. If
it is not free but is sparsely populated you can move the stuff out to free
it up or go to RIPE to get your /19 turned into an /18 (ie: get the /19
right after your current /19 [RIPE did give you the first /19 in a shorter
prefix block didn't they?]) in exchange for returning the /21 worth of
space [giving you 3 extra /21s worth of space], and divide that 2nd /19
into a /20, and 2 /21s giving them the 2nd /21 (still doing the same 2
announcements as you would if allocated out of your current /19).