The website sells hot tubs & swim spas. The company offers a standard payment plan - 10% deposit to secure the spa, then 70% payable 7 days before delivery with 20% balance due on delivery (or pay in full of course).
However for various reasons the delivery date can alter, so the due date for 2nd & 3rd (final) instalment will alter accordingly. This is why we would need to edit the payment plan after the initial deposit payment.
So far I have not found a plugin that allows this.