I emailed them a couple of weeks ago asking the same question. They got straight back to me by email within hours and sorted things out for me. I pay through "Go Cardless" and my revised subscription, which I doubled, begins in August. I received an email confirming that the old monthly payment was cancelled and another email confirming the revised payment.
The WS do need to make it much easier to join up, restart or increase payments, especially on the crest of this victorious wave.