Why take the chance? Just cancel the existing reservation and rebook it. Of course this means you have additional funds on hold on your C/C which is maybe what you're trying to avoid.

The 24 hour cancel is based on ticketing time. If you change your flights but a new ticket is not required then the original 24 hour cancellation time applies. If they have to issue a new ticket number then _possibly_ the 24 hour clock resets but I wouldn't bank on that because in the exchange the "method of payment" will show the previous ticket number. Anyone at United can see that and apply the 24 hour rule to it.

-RM