you are OK
you should be fine. as long as the area was underserved when you started working. INS regulations specifically allow this. you can check out the regulations on
www.shusterman.com. Click on "physician " section among the list ofr topics on the left side of the page and then check out the "physician NIW regulations". I am quoting them below
Will the Service Require a Physician To Relocate to Another Underserved Area If the Initial Area Loses Designation as an Underserved Area?
The interim rule does not require that a physician relocate to
another underserved area should the area the physician is practicing
full-time clinical medicine lose its designation as an underserved
area. The purpose of such a designation is to foster a greater
physician presence in underserved areas. The Service believes one of
the desired results of the statutory amendment is for physicians to
take up residency in these areas and become integral parts of the
community. Once an area is no longer designated as an underserved area, however, the Service can no longer grant national interest waivers for physicians to practice in that area (other than for physicians who will work in a VA facility).