Akershus and the Fantasmic package are never prepaid reservations. They are strictly credit card hold.
Be VERY careful booking CRT or dinner shows online with a dining plan, sometimes it works and sometimes it wants to charge you in full. Make sure you check on what it is charging your before you confirm that you want to book it.
CRT is prepaid in full because Disney wanted to make it more difficult for third parties to broker the reservations - awhile back there was a group on the Internet that did just that - booked up as many reservations as possible and then handed them out (by changing the name on the reservation) to whoever sent them the best story as to why they deserved one. This is probably also the reason why Disney no longer allows name changes on restaurant reservations.
By most reports, if you pay in full and then switch to DDP, there is one way to get an immediate refund - cancel the paid in full reservation and make a new one as a DDP user. If you can't get a new one, you would be keeping the current reservation and waiting to get refunded until you arrive at CRT or pick up your dinner show tickets.
If you don't have a DDP already on your reservation, you don't have much choice but to try and make the reservation by phone, because the online system has to recognize that you have a dining plan on your room reservation. As to whether the phone agent will allow you to book as a dining plan user when you don't actually have the dining plan yet, you would have to ask when you call.