It’s not possible to modify the Reference Application integrated forms. They automatically get reset to the default form if they are modified. As a work around, you can create a new “vitals” form, and copy the HTML code from the integrated form into your new form, which you can customize to your liking. There is not currently a way to disable/hide the integrated forms, so you end up with 2 “vitals” forms. See here for how to add your custom form…
https://wiki.openmrs.org/display/docs/Reference+Application+2.4#ReferenceApplication2.4-ManageForms
You would either need a new concept for Weight (lbs) or to enter Weight (lbs) on the form, and convert it to kg to be stored and calculated. PIH was looking at doing that at some point. Maybe @darius or @mogoodrich have more helpful info on what they ultimately did.