Can free Mosaic changes to another coterminal be completed online or do you need to call?

Points booking of original flight was FLL-DCA-BOS (B6-1680, B6-2754).

-Originally hoped to change to nonstop FLL-BOS (B6-770) but it was cancelled and replaced with a Mint aircraft (B6-8170). While there were seats available on 8170, it would not show up as choice for SDFC.

-Also searched for FLL-PVD (B6-1196) in the online change tool and was able to find it and select seats but on the final screen it wanted $496 additional payment.

-In the end, chose FLL-BOS (B6-770) which was no issue except for being very full and unable to get 2 seats together.

Anybody have success switching to a coterminal online or is expected to require a call?