After a supply has been used - in your case because of an imported job, the option to change the purchase unit, cannot be changed. If you need to get the purchase unit corrected, you can rename this original supply name from the Input Tab-Supplies-Seed-(Crop name). Right click on the seed variety name with wrong purchase unit and select Properties. Add the letter Z in front of original description. Click OK. Void the imported Jobs (only if you have access to the orignal data files) and import through read job data again. Then when creating those new supplies - make sure you setup the Purchase unit for desired choice.