Or, if you're using Photoshop or other image editor that supports 16-bit channels, just get a DEM for the same areas of the world that you used for your updated map and make a 16bit grayscale map using the same DEM parts. It's the same process that you did for the updated map (chop the mountain parts that you want and paste those as layers on the new terrain). The final finishing would be to adjust the DEM levels until they met your desired profile.