Hey Ed.
There is not a way to do this within the InfoPath UI as you have seen.
You could export your source files, and in your manifest.xsf cut and paste between the two, but obviously this can be problematic making sure your syntax is the same, etc. Depending on how many rules it might might be worth it and it might not.
You can paste your formulas across, if your data source is the same and fields are titles the same.
Another option is to base your new form on the old one and do a save as and then take out wht you don't need but keep what you do.