Workflow for Replacing DOM With DEM Values in a Zone Specified by Vector Layer

1. Create polygon file with same CRS as DOM and DEM and draw polygon mask (obv. polygons with holes don't work!) 

2. Use the "Rasterize (overwrite with fixed value)" algorithm, using the polygon mask as vector layer and the DOM as raster layer (set the DOM as reference layer!). Set the burn value to -9999. BEAWARE: The original file will be overwritten - if you don't want to alter the original file, make a copy of the DOM first!

3. Use the raster calculator to rewrite the values in the DEM with values from the DOM by using this formula: 

(DOM@1 = -9999) * DGM@1 + (DOM@1 != -9999) * DOM@1