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If you bought from intuit, call them and you should just have to pay the difference. Be aware that enterprise is subscription only, unlike pro and premier.
Intuit does not do discounts, or preferred pricing for upgrading to a new version, you buy it again, or in the case of enterprise continue to pay the monthly fee.
If you don't want or need Enterprise, an option is to have your file converted to Pro / Premier format to use it there. Use our BRC Enterprise to Pro/Premier Conversion Service for a seamless conversion of all of your data.