...I have been doing this in leiu of the find and replace macro...
Make sure the file you are working on is the only file open in Corel. Goto TOOLS and open the Color styles docker. Drag ALL of the image into the docker, and a list of colors in the graphic will show up. From there you can drag and drop colors from the pallette you want to use over the ones you want to replace, and they will change.
When I am done, I will usuallly recheck it by doing this: delete all the colors from the Color Styles docker (it wont change the graphic), then close it. Reopen it, and drag the image in again. Should only list the colors you want now. If any color comes up again that shouldnt be, just repeat the dragging and dropping.
...I use this to convert all colors in a file to the PMS pallette colors, which makes separating a bit easier. Its also cool for doing vector underbases, Just print all your color films, then drag and replace the those colors with PMS Trans White, then print the Trans White Sep.
...Just another way to catch a mouse. Hope this helps!