and I am sure it will work, and not be too, if at all, tedious.

I use a lot GIS software with imports from soil labs, and set up my own edits for updated rates, automate them to load in CSV files, and import them back to my GIS, then export them as edited back to VRA controller files. This way my growers can get my or somebody else's recommendations, likely an improved, customized improvement over that of the soil lab's.

You can take the data from the DBF component of the shape file, dump or save as Excel or other SS file. Then, you can build formulas to adjust your own rates accordingly to a new column in the SS file. Save the file as a CSV, import to the GIS, and then export a VRA controller file from there.