Placeholder benefits cannot be added to an already booked cruise. The deposit funds themselves can be, but you would lose the OBB benefits.
We had a summer Alaska cruise booked, with placeholder/OBB benefits. We decided as a family that even if it sailed we did not feel it safe to at this time so we would move it to 2022.
The day before Canada extended its cruise ship ban, I called
DCL and
moved the cruise to a dummy Disney Cream cruise for late May. I was very clear in explaining what I was doing, that the Dream cruise was just to hold our deposit funds with DCL until the summer 2022 cruises are available to book. Because the Dream cruise is a fraction of the cost of the Alaska cruise, no further deposit was needed.
They extended the placeholder until May 31. This may not be far enough for us for Alaska 2022, but I will ask for another extension when it comes to booking the actual cruise and see what they say.
We did the move to a dummy date for a few reasons: we wanted to preserve the OBB benefits; and we are Canadian, so there are foreign exchange rates and fees involved that mean that if our deposit is refunded we lose money overall. Since we plan to cruise again with DCL and relatively soon, I am fine with DCL keeping hold of our deposit funds. OTOH, I was NOT willing to PIF even though that would mean we would get the FCC when the Alaska cruise is canceled, as that for me was too much money for Disney to have right now.
Note that if you CANCEL a cruise booked with placeholder benefits you LOSE those benefits and can't get them back.
If you want to keep the OBB benefits, then what you want to do is
MOVE the cruise to a new sailing. You will keep the same reservation number. The placeholder benefits will be retained as long as you are within their use window [or if applicable any extension of that window that DCL is giving right now].
Note that other rules can come into play, such as if you are past PIF date, so if that applies make sure you understand the implications.