Hi Susan, welcome to the ZenLedger community!
Great questions - here’s some guidance.
If all you did in 2017 and 2018 is BUY with fiat currency (such as US Dollars) and hold or transfer to wallets, then you would not have had any taxable events in those years for your cryptocurrency holdings. Sales and Trades are, in most cases, taxable events according to current guidance from the IRS, but buying with fiat and holding are not taxable events.
If you TRADED one coin for another (such as buying ETH with BTC), you would have had a taxable event and it would be a good idea to refile your return. Receiving airdrops is not taxable either, unless you sell the airdropped coins. Read more about the current guidance on taxable implications of cryptocurrency here: How will Cryptocurrency Investments affect my taxes?
With ZenLedger, the information you imported to produce your 2019 tax report should also be good to refile any other year - you’d just pay for the reports for that year.
Please let me know if you have any more questions, and welcome to the community!