Hello @rcppubs23,
Thank you for reaching out to the Community! If you see that our default field names and your custom field names aren't appearing or not allowing you to select them, that means that they are already being used to label a column in that spreadsheet and you will want to create a new custom field. Also, when you upload existing contacts back into the account, you won't lose any data that is in Constant Contact, but you are able to add more information to each email address if you want. So it sounds like you should be just fine to proceed without losing any of your data.
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