I have noted that some Heckel keeper's find their fish will only eat one or very few foods. Ths is not just a Heckel phenomena. All fish become habituated to the most available food source. This is only a temporary thing. Your Heckels will after some period of transition, accept a wide variety of foods.
Case in point. There are numerous volumes dedicated to the selective feeding of trout, even bearing titles such as Selective Trout.
This is common to most fish. One should make every effort to broaden the base diet of the discus. feeding decreasing amounts of the habituated food along with other foods is the easiest way to accomplish this.
Larry Waybright