Booking a room before free dining is offered provides you no advantage whatsoever with regard to getting free dining if and when it is offered.
Regarding whether or not you can switch your reservation later:
It does depend on the rules applicable to your current reservation, with whom you made the reservation, and whether there is availability in the limited "free dining" inventory at the time you call to switch your reservation.
If you or your TA didn't make your reservation with Disney directly or with
AAA, then you probably won't be able to switch your reservation. Free dining has never been offered via
travel agency allocations (except AAA), so if your current reservation is from a travel agency allocation, your travel agency will generally not allow you to switch out of their allocation, at least not without paying a fee or forfeiting a deposit.
Also, if you couldn't actually reserve free dining in the room you've already reserved for the dates you've already reserved, then you cannot switch your existing reservation to free dining.
Keep in mind that Disney has already offered free dining for 2007 twice, but both times it was to targeted groups. We don't know whether or not they'll offer free dining again this year, of course, but we also don't know, if they do offer free dining again, whether or not they'll offer it solely to targeted groups.