When you purchase a ZenLedger plan for 2019 (or any other year), you'll pay once for access to that year's reports. The plan includes unlimited, infinite, forever access to the reports for that tax year.
Think of it as similar to TurboTax's business model. You're paying for the completed reports, not for the use of ZenLedger software.
To illustrate, here is an example:
It's April 14th, 2021. Time to get my completed tax return out the door, so I buy a plan for the 2020 Tax Reports.
In 2023 (or any year, for the rest of time), I realize that my 2019 Tax Return was missing the coins I held on my paper wallet.
In 2023, I can log back into ZenLedger and edit my 2019 reports for an amended return. There will be no extra cost and no surcharges, assuming the changes to the return don't push me into a different plan tier.