Robin, you wont be able to change the field name in your database, but you can change how it is displayed on your website. Create a custom theme and at the top of each theme page, do a search for "member" and "membership." Instances of "member" enclosed in liquid tags, such as {{ request.current_user.membership_renewal_info }}, won't be visible to website visitors. You can then change the words in white - white text in a template displays on the public website and you can change it to whatever you want it to read. 

Additional information on creating a custom theme is available in our theme documentation.