To keep a backup or archive of your calendar's historical data, you can export your data on a regular basis.
If you need to access more historical data than your current subscription limit, you can upgrade to access more retained historical data and export it. Then do regular data exports to keep your historical data records complete.
1: Temporary subscription upgrade
You can upgrade your subscription once a year (or however often makes sense for you) and pay the monthly rate. You'll be able to access more historical data on the upgraded plan, which you can then export. That way you only pay for a single month of the upgraded subscription.
Be sure to consider retention limits if you use this method. Upgrading a subscription plan provides immediate access to any retained historical data that falls within the historical data limit of the upgraded plan.
2: Regular data export
You can export your calendar data at any time. To keep complete records of historical data without upgrading, export it on a regular basis:
Then set a reminder to export all calendar data on a regular basis (e.g. monthly, quarterly, or annually). If you keep up with these regularly scheduled exports, your backups will always be done within the allotted historical data and you won't need to upgrade for the exporting process again.
Note that you can also periodically save your calendar to a PDF file via the 'Print & Export PDF' functionality.
Combine these methods: Upgrade to export all retained historical data needed up to the present. Then set a reminder to export data on a regular basis before it becomes inaccessible.