Hello,
"I am forwarding a work item to a different user, but the old approver is receiving reminders"
Reminders from where - the workflow or Extended Notifications?
The most likely scenario, as mentioned by others, is that the notifications are being sent by the workflow and who it thinks the Approver is doesn't get updated when there's a Forward. This is another reason to use Extended Notifications instead of putting Mail steps in the workflow.
regards
Rick Bakker