The primary reference for the transfer of government property to contractors is DFARS 245.  DoDI 5000.64 offers policy guidance and references for all types of transfers. See also DFARS 252 for details on accountability questions.  DoDI 4161.02 covers documentation requirements for transfers.

DFARS 245 states the Standard Form 30 is to be used to document transfers to an existing contractor - such as your modernization work for your training devices. The DD Form 1423 (Contract Deliverables Requirements List) will document the "transaction" supporting the contract.  The DD Form 250 is used to complete the transaction and pay the contractor for the successful return of the upgraded asset.

Per DoD 5220.22-M this process can be applied to classified items as well.  Additional protocols for classified item security in transit, safe-keeping at the contractor site, and regular physical inventory requirements would be part of the contract.