Which version of EPiServer 7? I tried with two different sites on 7.0.586.1 (initial release) and 7.0.586.8 (patch 2) but could not reproduce this behaviour. When changing the Field Name (and even when changing the property Name) the value is not deleted.
Are you sure you didn't change the property TYPE? That might lead to the value being deleted.
I am using EPiServer 7. When I change the field name of a property in admin mode, the property's value get deleted in edit mode.