[Eee Pad Transformer Q&A, Help & Troubleshooting] luna_c666:
I am fairly certain Asus like most device manufacturers try to keep you from being able to downgrade from official OEM versions of software back to older versions- the reason being, of course, that the newer versions sometimes fix software and security vulnerabilities (like 8.6.5.21 losing the ability to use an APK file to root, like razorclaw, and imagine a rogue application exploiting this vulnerability, achieving root, and broadcasting to the developer any information without notifying you about it..). The interesting thing is that the method of denying firmware 'updates' to downgrade seems to be arithmetically calculated from the version numbers, basically if update package version is < current version update will be denied. However, when they decided to engineer the update denial this way, it left it's own vulnerability, in that firmwares for the TF101G, being the later model of the TF101, automatically have a higher firmware version because of the 3rd digit is a 6 instead of a 5, indicating that the version is for TF101G. This left the old honeycomb ROMs with this vulnerability- if you had updated to 8.6.5.21, you were still able to 'update' to an older firmware the device viewed as 'newer' than the current device firmware, i.e. 8.6.5.21 users could update from a stock device to downgrade their firmware using the TF101G firmware 8.6.6.19, which could still be rooted with the APK vulnerability.

Now that we have moved onto ICS and there is not yet a TF101G version of ICS available, we lack this native downgrade option, and we are yet to see if Asus is going to fix this 'vulnerability' by simply amending the TF101G and TF101 version numbers, or by altering the way in which the OEM recovery mode accepts or rejects Asus update packages, I have the feeling that they already have with the 9.2.1.11 update recovery that was included.

All of the precursor information I just listed brings me to the only current method known to rollback your firmware if you are on ICS, either 9.2.1.17 or 9.2.1.11..you must root your device with ViperMOD, install CWM via RecoveryInstaller, and use CWM to manually install the firmware to a previous version. This will perhaps add a few layers of complexity..you may have to toggle signature verification and script asserts enabled (I am making a wild guess on this one, I've not yet tried to downgrade just yet), and you will be persistently bothered by Asus update notifications (there are plenty of threads describing how to rid yourself of these). My guess is that soon Asus will begin pulling the HC ROMs from their official website so you might want to grab that ASAP. And if you want to keep from getting bothered by update notifications yet stay stock 100%, you can (until they release the ICS for it) flash the TF101G version of the firmware, 8.6.6.21.

IN SUMMARY; to downgrade ICS to HC, you will have to root and flash recovery modes. There doesn't seem to be a way around it. And IMO it's probably best to NVFlash back to HC, no telling what kind of data will be left that will conflict with HC from ICS if you do simply flash a HC ROM via CWM. You will probably want to make sure you have the right kernel just in case you have to flash it as well..I can imagine a nightmare scenario if the Asus update didn't include a kernel that it leaves your ICS kernel intact, which will not load HC, and also flashes over your recovery, leaving you pretty much bricked without NVFlash. Fairly sure Asus update contain the full ROM- kernel included, but I always err on the side of caution..I'd flash the HC kernel first personally..just to be safe. If nightmare scenario does occur, you can't flash it after you reboot.