I see that you are relatively new so I understand the confusion with some of the terminology, but it makes your situation a little unclear.

I'm guessing that you purchased 3 award tickets in the main cabin. Did you select seats at that time? The next question would be if Delta switched planes after your purchase because they don't normally just change seats (there are some circumstances). If your seats got switched you should call Delta to see if you can get them back or similar seats that are acceptable.

I'm also guessing that you are asking about buying seats in what Delta now call the Delta Comfort+ cabin which Delta calls an upgrade. If seats are available, Delta generally makes them available for purchase and you essentially keep your same ticket. A price will often show up on your reservation if you view it online or you can call and see what the price would be.

I'm making some assumptions above so I don't know if I described your situation. If you would supply some additional details, someone will be happy to respond to your question.