You can not purchase the dining plan, let alone get a
free dining discount unless you are booked with a Magic Your Way Package that includes room, tickets and dining. There is a 2 day base ticket minimum requirement to get the free dining plan discount.
Your TA doesn't seem to know anything about Dining Reservations at WDW. You do not have to switch your reservations and it will not mess up your dining reservations to apply a free dining discount to your reservation. You don't even have to have a WDW reservation to make dining reservations. You wouldn't do anything to your dining reservations if you happen to get a free dining discount. You can ask that the reservations CM link your ADRs to your reservation number, but that isn't required either.
If you are booked under a room only reservation right now, that room will have to be cancelled and you will have to rebook under a package reservation.