No, the 40G Transceivers used in the 8400/8320 are single speed only.
If used in another QSFP+ or QSFP28 port, only if that QSFP28 port is designed to also support 40G speeds will the 40G Xcvrs work.
The 8400 100G QSFP28 ports on the JL366A Aruba 8400X 6p 40G/100G QSFP28 Adv Module can operate with a 40G Aruba Xcvr inserted (JH231A, JH232A, JH233A, JL308A and the 40G DACs JH234A, JH235A, JH236A).
See the "ArubaOS-Switch and ArubaOS-CX Transceiver Guide" for more details and compatibility information. (make sure the guide you're looking at has "ArubaOS-CX" in the title -- the latest edition as of July, 2018 is Edition 4.