Here are a few considerations.
Regarding the memo, it should include at least what, why, and when, with the "what/why" explaining that the P-card is replacing petty cash, and the "when" identifying the deadline for turning in the cash and records/documentation on balances.
If there is a specific designated petty cash custodian, that person as well as the responsible business unit manager should be addressed. Your memo should include information on the final reconciliation, how/where to deposit the funds, and how to notify the proper authorities that it has been deposited, etc.
Then there should be some work in AP closing down the reimbursement process as well, so AP knows how to proceed.
In the case of one of our experts, some of their locations' ideas of a solution to shutting down petty case was to go out and spend it all so there would be nothing left to deposit! (Pizza parties, etc.)
If you want to prevent that from happening (because it became kind of an uncontrolled free-for-all), you might want to be specific about re-depositing the funds, not spending them down—assuming the P-card program is already in place.