First, make sure the current recurring fee for the package is not under a Paypal subscription. If it is, make sure to cancel the Paypal subscription before continue.
If you are using Clientexec 5.9.0 or higher, it now has the ability to merge packages using a multi-select dropdown in the merge section to select the packages you want to move to the other account.
If you have a lower version of Clientexec, then use queries like these ones to change the package owner and the recurring fee owner:
UPDATE `domains` SET `CustomerID` = new_customer_id WHERE `CustomerID` = old_customer_id AND `id` = package_id;
UPDATE `recurringfee` SET `customerid` = new_customer_id WHERE `customerid` = old_customer_id AND `appliestoid` = package_id;
Replace new_customer_id with the id of the new customer.
Replace old_customer_id with the id of the old customer.
Replace package_id with the id of the package you are modifying.