If your Army List allows you to upgrade 1/2 of the troops, such as experienced bow to skilled bow and you have 12 bases of light horse ( two units of 6 ) do you get one unit of six that are skilled and one of experienced?? Or if you army list allows 36 light horse and 1/2 can be upgraded , or can you upgrade 18 and can choose not to have any experienced ?? I guess this would apply to all type of upgrades.
In such cases you get to upgrade half of the number you have chosen for your list - so half will be the upgraded types and half not upgraded.
Were it to be intended differently the upgrade would be of the form 0 to X.