What happen if I switch those labels when generating Artwork
The fabricator doesn't necessarily care what you call the layers.  They are going to build the PCB in the layer order you tell them and based on the gerber files named what you call out.  Now, obviously naming something "TOP" that you want on the bottom is going to create confusion.  You have the option to mirror the particular film when generating the artwork.  My recommendation would be to generate the artwork until you get what you want in the gerber view (use and independent gerber viewer to verify/validate), and then rename the output files to match what you want prior to packaging for sending to the fabricator.  This is obviously a little bit of a hack, and now your design files won't match the gerbers exactly, but unless you have the ability to swap the layers around in the tool (I am an Altium user and don't have enough experience to comment on OrCAD), I can't think of a better way to do this.  Just remember, the fabricator will build it how you tell them to build it based on the artwork you provide.