Does editing a product change the License File?

I’d like to add another custom field to an existing Product. Will this modify the Product.dat or Version GUID? Will I need to rebuild the app with a new Product.dat?

When you create a product version, GUID gets fixed, no matter what you change in the product version, GUID will never change unless you delete the product version itself.

The only change made in the product version  which requires client side update is the trial length.

1. If you are using unverified trials - then you need to replace the old Product.dat file with the new one, and update the same in your code - which means you need to recompile.
2. If you are using verified trial - then you only need to replace the old Product.dat, no need of recompiling.