This gives the pair the ability to choose. If they refuse entry into the new box and use their original one, I would rethink the reasoning behind the nest box change. I have been breeding birds. I've been told and have read that the controversy existed long before, and I have no doubt it will continue for years to come. Products for birds are really not any different than the products we buy for our own everyday use.
Individuals prefer different brands, types, flavors, colors and so on.
I don't think we will ever see the day when a company states that its product isn't any better or different than another. At this time, regardless of the diet your green-cheeked conures have been fed, I recommend that you duplicate the diet they were on before you got them. If it is a diet that you are comfortable with and there are no dangers to your birds, I would continue with that diet. If it is a diet that is nonvaried, add veggies and greens. It would be a good idea to question the prior owners about any special foods given to the pair during breeding season and when they were feeding their young.