It's nice to see you in the QuickBooks Community today. I'd be glad to help you get back on track so you can modify the service item from the timesheet entries.
Yes, you're correct. In the timesheet drop-down, it doesn't show the entries dated more than a year ago. However, you can run the Time Activities by Customer Detail report to modify the service item in the timesheet.
On the left pane, select Reports.
Find Time Activities by Customer Detail.
Filter the date.
Find and click the entry.
Select the correct service item in the Service drop-down.
Click Save and close.
That should do it. Let me know if you have any other questions about modifying the service item in the timesheet. I'll be happy to help you out.