If I understand this passage correctly, this is about the Argentinian wealthy not wanting to share the beach with their economic inferiors. Thus, something that gave new "access" to the beach for those otherwise excluded led the wealthy to move out. The word "migrar" is particularly poignant here in that it captures that notion of a mass and almost instinctive movement by the group, similar to the way birds migrate south in the Winter (or, I imagine, north if one happens to be in Argentina).