Below downgrade was working for me:Cisco 3850 downgrade from IOS version 16.x to 03-07.xFirst :copy cat3k_caa-universalk9.SPA.03.07.05.E.152-3.E5.bin to root of usb stick copy cat3k_caa-universalk9.SPA.03.07.05.E.152-3.E5.bin to flash:place usbstick ...

What i did to solved this issue : remove the port-channel on the suspended interface on one site , shutdown both ports , no shut the port suspended, you see the port up and running again, now shut the port again, enable port-channel again on the port...