Fast and easy conversion of a .reg file with image coordinates (x,y positions) to sky coordinates (RA,Dec;sexagesimal):
ds9 image.fits -regions load input_imageCoordinates.reg -regions system wcs -regions skyformat sexagesimal -regions save outout_skyCoordinates.reg -exit
with automatic exit of the program!
Thanks to Alexandros M. for providing some examples!
References:
[1] ds9: http://ds9.si.edu/site/Home.html
[2] ds9 command list: http://ds9.si.edu/ref/command.html