locking a field in a form

Hello,

I want to be able to lock a field but still allow the user to enter NEW data
into the field in the form. (In other words, allow users to enter new data
into a particular field of a form but not change existing data in that
field). If I lock the field in the properties sheet, the data can't be
edited, which is good, but you can't enter new data either, which defeats the
purpose of the form. Is there a way to do this?

Thanks,
Debbie