Hi @spotlight_sound,
Depending on how many options your check list has you could also work with filtered views and bulk edits. You’d navigate to Menu > Contacts > Contacts > More filters, search for your property, set the filter to “[Property] is any of Supplier”, select all returned contacts and bulk edit them. You’d have to do this one by one, for each option of your property. Feasible if there are 5 options. Not so feasible if there are 50.
Your solution works perfectly well, too. If you export your contacts, they will come with the HubSpot contact ID. If you keep this column, you’ll be able to tell HubSpot during the import file to use it as a unique identifier. (This is a bit more reliable than using the email address as unique identifier, as you mentioned there could be contacts without an email address. Per default, each contact has a contact ID in HubSpot.)
To make sure that you’re not accidentally updating any other values (if, for example, a contact has filled out a website form while you made the edits in Excel which you’d be overwriting with your import), I’d recommend only importing two columns: the HubSpot contact ID and the column for your custom property. HubSpot will each contact based on the HubSpot contact ID with the new value in the custom property column.
Hope this helps!