You can change the field type from 'Number' to 'Lookup List' without losing any of the previous values.  Please note that if you change the type back to 'Number' after changing it to ‘Lookup List’, you will lose all the previous values, including the values that were populated when it was a ‘Number’ type field. To prevent any data loss, I recommend that you either make a backup of the project or export the data to a spreadsheet before changing the field type.