What version of Sage 50 2017 did you buy? Perhaps a few more details as to any error messages that you get when you try to restore the data file that you got from your accountant might help in providing assistance. What OS are you using?

We bought Sage 50 Pro Accounting 217, Installed it on a brand spanking new Asus running Windows 10. The accountant gave us the files in his 2016 version of Sage. The error message states that it cannot be opened from this version.

There are three Editions of Sage 50 - Pro, Premium, and Quantum.

They will upDATE from older VERSIONs (i.e. 2017.1 will update from 2017.0 or 2016.3, etc.)

They will up-GRADE from the same or lower 'EDITIONS' Pro-> Pro; Pro->Premium; Premium -> Quantum

What is the exact wording of the error message?

This company has been converted to Sage 50 Premium Accounting. You can no longer open it with this version of Sage 50.

Sigh - Do I have to upgrade my version too?

If you want to open that file, you'll need at least Sage 50 Premium Accounting.

Your accountant may have set up your company books in the Premium Edition because that's what you need in terms of features or limitations of the 'Pro' Edition. Or because that's the one he / she usually uses.

'downgrading' to the 'Pro' Edition may be an option, but requires the data to be converted (expensive), or the company books to be set up again (expensive). It would be pointless to consider if you need features of the 'Premium' Edition.

Don't need Premium Edition, just happens to be what the accountant is using. I can set up the company books again myself if I have to. Just looking for an easier way. How can I downgrade the data file to the Pro Edition? Time is of the essence. GST is due the end of this month and I have from Oct 2016 to present to input all over again. Accountants data file is from our year ending Sept 30 2016.

Downgrade is basically one of two ways:
Search the Internet for 'downgrade Sage 50 pro from premium' and hope you find someone reputable to do it. There used to be some posts on this site.

Install a trial version or ask someone who has the matching software to take your Accountant's data file, and From the File menu, export all the 'item lists' / 'Ledgers' into the Sage 50 file import format, then get the software you're going to use set up and ready, so that you can import each 'item list'. Once that's all done, you can set up the opening balances (last year's closing balances) and then start putting in entries.

You could also ask your Accountant whether they could provide the data in a format you can use.