If you copy data from another source and paste that data directly into an excel spreadsheet, you will likely change the format of the cell while you are pasting the data. Instead, use the "paste special" function, and paste values or text only.
If the structure of a cell has changed, or some other change to a spreadsheet has occurred in the spreadsheet prior to upload, you may get the following error message:
"Your data could not be imported for the following reason(s):
The file you uploaded is not the original import file, or has been structurally altered. Please download a fresh import_products.xlsx file and try the import again."
If you get that error, download a new import spreadsheet, copy your entire spreadsheet, and instead of "paste" to the new spreadsheet, use "paste special" and paste only the values (or text) – then the fresh spreadsheet should upload.