Laying a flagstone patio over two different surfaces

I'm installing a flagstone patio to replace an old concrete one that's about 8 inches + thick. The old patio butts up against my back door and runs 20 ft about 1/6 inch from the house/foundation. We have already removed a good portion of the old patio with a jackhammer. My brother thinks we should leave about a 3 ft. width of the concrete patio closest to the house, and lay the flagstone over it and out into our gravel base to make the new, larger patio. My concern is that the portion of the patio NOT on the concrete (which will be the majority) will shift and sink and that that could eventually cause problems with buckling, breaking or even drainage at the junction of concrete and gravel base.

Thank you very much!

my email is audramelt@yahoo.com