I was dealing with a similar situation and I want to acknowledge people who recently posted here about that kind of issue, and especially @elletaylor for this tip. I just booked a solo vacation in mid-December and I could only find a Candlelight package for 3-4 people (at Coral Reef). So I booked it with the intention to change my party size as I typically do with ADR, but there was no “change“ button, just the “cancel” button.
I used the chat function. The WDW agent asked me to provide information through a chat-based form s/he sent to me, and the party size was successfully modified from 3 to 1.
It will be an opportunity to have a first experience at Coral Reef where I have never eaten... and to have a seat for the Candlelight Procession with Neil P. Harris as the narrator...