Yes, Beckett, the Labour party did offer reduced subscription fees to new members. However, real change in the Labour party came from grassroots activists in the trade union movement who were affiliated to the party through the broader Labour movement in the country.
They were at the base of the formation of Momentum, and many had also directly held longtime affiliation through their trade union branches in many District and Constituency Labour parties.
They became much more active and determined to bring about change when after twelve years of Blairite government those activists felt they had been totally ignored by that administration.
So, change was brought about starting at District and Constituency level, and the rest is now history.
The situation with the current infiltration of the Conservative party is I feel much different, and much more sinister.