That's an enormously large "if": and, given the total absence of democracy within the Labour Party now, I can't see it happening - the "Warwick Agreement" between unions and Labour leadership proposes absolutely minimal reforms, and there is little reason to suppose the leadership will stick to it. Compare, for instance, Old Labour's treatment, when in government, of the 1975-1979 "Social Contract", in many ways a similar pledge: the unions carried out their half of the bargain to clamp down on wage militancy, the government fulfilled their commitment to increased public service spending by introducing the biggest round of spending cuts since 1945.

In addition, it's not just nice lefty types wanting people to join the Labour Party - see here, for instance.