Concatenate into Field in Update
You can concatenate text with text, so you need to CAST(). For example,

notes = CAST(GETDATE() AS varchar(max)) + 'Old-' + CAST(ISNULL(notes, '') AS varchar(max)) + 'New-' + CAST(@Quantity AS varchar(max))


Note that concatenate null with something will give you null, so to implement CONCAT() function's functionality you need ISNULL().