Before 1989, only members of the current Worldcon were allowed to nominate for the Hugo Awards.
The first year that members of the previous Worldcon were allowed to nominate for the Hugo Awards was 1989. (The change was ratified in 1988).
The first year that members of the subsequent Worldcon were allowed to nominate for the Hugo Awards was 2012. (The change was ratified in 2011).
Remember that each individual can only nominate once, even if that person is a member of two or three of the Worldcons whose members are eligible to nominate.
I do not give a lot of credence to theories that broadening the nominating franchise to cover a three year span of WSFS members is the main cause of the recent increase in participation. If anything, I think it has been the result of a relatively long slow plod of promotion of the Awards, along with an increasing number of people paying attention to how they can actually participate, instead of just assuming that a Mysterious Them gave out the Awards.
I am, however, somewhat bemused that somewhere along the way, we went from "$40 just to vote? Highway robbery! Unfair!" to "Only $40 to be able to help decide who gets the Hugo Awards? What a bargain!"