After you file

That's false.  Where did you see such?

TurboTax has saved online returns for years.   At some point in time, if people don't keep using a paid product, access expires, but the returns do not disappear.   They can pay for a current product to unlock the access.   See this FAQ:
<a rel="nofollow" target="_blank" href="https://ttlc.intuit.com/questions/3355524-when-does-access-to-my-prior-year-turbotax-online-returns-...>

That having been said, it's best if people always make their own backup copies each year,  and store them safely and securely against computer theft, hacking, etc.   Then they don't lose access to them.

I'll tell you below how to contact TurboTax Support so you can tell them you bought the 2017 software.