Will changing a category on an already reconciled transaction (dating back a few years) mess up my records?

I can share some information with reconciliation, @d_kaplan.

Yes, you can recategorize your transactions in QuickBooks Online (QBO). This won't affect their reconciliation status unless you'll edit their dates, amounts, and statuses.

You can update the category from the Categorized section of your Banking tab. You'll just have to Undo the transaction to move it in the For Review tab and set the right category from there. 

Here's how:

1. Go to your Banking tab and proceed to the Categorized section. 
2. Go back to the For Review tab and find that transaction.
3. Click the Undo button under the ACTION column of the transaction that you'd like to recategorize.
4. Go to the For Review tab, look for those transactions and click to expand it
5. Set the right category in the Category box.
6. Select Add once you're done. 

Here's an article that helps in handling your banking transaction in QBO: Categorize and match online bank transactions in QuickBooks Online. 

This article will guide you in resolving issues you'll possibly encounter when reconciling your accounts: Fix issues at the end of a reconciliation in QuickBooks Online.

Let me know if there’s anything else that I can assist with about your bank transactions by leaving a comment below. I’m always here to assist. Have a wonderful day!