If you export a GEDCOM file from FTB,
and then import it as a GEDOM to the Family Site,
using the Family Tree > Import GEDCOM page,
you won't have any trouble updating it online.
That being said, when you publish an FTB project to your family site through FTB,
It is not editable online, because this will require a two way synch mechanism between FTB and the online Family Site.
We don't have this yet, but we are planning to add this feature for FTB 6.
Noam \ MyHeritage Team