But I agree, it's a bit poor not making the change more public to existing customers
Not
"a bit poor" but
IMO "very poor"
There were comments on here at the time, and perhaps elsewhere, urging PlusNet do the honest thing, and apply the lower charge to to all affected customers.
There was no excuse other than being money grabbing for PlusNet to
not let existing customers know. Probably someone in the Finance department said "legally we can continue to charge what had been agreed" and the PN staff had no option but to follow the company line, whereas if there had been a change in the opposite direction, they'd have no choice but to make customers aware they were going to charge them more, and no hesitation on making the change with a hard line of "we informed customers of the change in fee, in advance of applying that change".
It smacks of appalling customer service, and it stinks, because only PN themselves can know exactly how many customers are still paying over the odds. In theory, someone who has referral income could send an unsolicited "warning" e-mail to those whose usernames are listed on their report and could make them aware, but at the same time, not all recipients would be affected, and it would then be junk mail to the majority.