This was already partially answered by Tim in http://community.arubanetworks.com/t5/Unified-Wired-Wireless-Access/Downgrade-Aruba-7205-Controller-to-6-4-2-3/td-p/254104 -- min supported OS for 7205 is 6.4.3.0.
But to get back to the question, as you saw there is no boot partition selection in the cpxboot, to change it you need to wait for cpboot which is the second stage. Both have very fast countdowns, just 2 seconds.
Once you interrupt the cpboot, to see the installed versions use "osinfo", e.g.
cpboot> osinfo
USB0: setting speed to USB_SPEED_HIGH
2 USB Device(s) found
#1 Storage Device(s) found
Partition 0:
Reading image........................................done
image type: 0
machine type: 28
size: 82100180
version: 6.4.3.3
build string: ArubaOS version 6.4.3.3 for A70xx (p4build@cyprus) (gcc version 4.4.5) #50787 SMP PREEMPT Fri Jul 10 04:51:15 PDT 2015
flags:
oem: aruba
Image is signed; verifying checksum...
passed
Signer Cert OK
Policy Cert OK
RSA signature verified.
image verify: PASS
Partition 1:
Reading image................................................done
image type: 0
machine type: 28
size: 99958804
version: 6.4.3.6
build string: ArubaOS version 6.4.3.6 for A70xx (p4build@corfu) (gcc version 4.4.5) #52485 SMP PREEMPT Wed Nov 11 15:32:42 PST 2015
flags:
oem: aruba
Image is signed; verifying checksum...
passed
Signer Cert OK
Policy Cert OK
RSA signature verified.
image verify: PASS
cpboot>
then, to switch partitions we can use the def_part command, where we can select partition 0 using "def_part 0" and partition 1 using "def_part 1".
Now start the boot using the command "bootf" and verify that the correct partition is booting, look for the string "Loading image 0:x" where x is the number passed to def_part.
For example, here we see partition 1 is selected and it boots from 0:1 as expected.
cpboot> def_part 1
. done
Writing NVRAM to Flash... ffffffff8bd03858 ffffffffb6220000
done
cpboot>
cpboot> bootf
USB0: setting speed to USB_SPEED_HIGH
2 USB Device(s) found
#1 Storage Device(s) found
##Loading image 0:1########################
Image is signed; verifying checksum...
passed
Signer Cert OK
Policy Cert OK
RSA signature verified.
[ 0.000000] 0:xlp_napi_vc_mask 0xf
[ 0.000000] 0:sae frequency is 250