Simply select the new units in the units drop down next to the Title field. This doesn’t change any of the nutrition values.
Once you've started your recipe in grams or kilograms, percentages cannot be entered. If you click on Percent (%) in the dropdown, the percentages of the ingredients in the recipe are calculated from the quantities.
If you need to enter a recipe in percentages, you must select Percent (%) in the dropdown before you start adding ingredients.
If you select Kilograms (kg) in the dropdown, when you select your ingredient, kg will be the default option shown. However, you can still select grams for an ingredient, if you wish. It is fine to enter a mixture of grams and kilograms, if that's how you measure them.
You can also select litres, millilitres etc for liquids.