Cisco document instructs disabling "fpd auto-upgrade" when upgrading from 5.3.1 to 6.1.x

[shmohsin1]

https://www.cisco.com/web/Cisco_IOS_XR_Software/pdf/ASR9K_Upgrade_Downgrade_Procedure_IOSXR_Rel_612.pdf

This will significantly impact the down time of the router during the upgrade.

• What is the reason for this? 
• Is there a significant risk of card failure if fpd auto-upgrade is in place? Why is it more risk to fpd upgrade with fpd auto-upgrade than by doing it after the 6.1.x upgrade. Aren't they the same fpd files that will be loaded on to the cards?
• Is it not riskier for some card to fail with two reboots instead of one? 

A quick sincere answer from Cisco is appreciated.

Thanks!

[Aleksandar Vidakovic]

If you execute the FPD upgrade with 6.1.x, it will complete significantly faster than if you execute it on 5.3.x. This is due to improvements made in 6.x.x. You can read mode about this in BRKSPG-2904 from Berlin 2017 or Las Vegas 2016.

In addition to that, if you run the FPD upgrade with 5.3.x, some cards may reach a timeout (see https://supportforums.cisco.com/document/13116771/fpd-101-asr9k-platform).

/Aleksandar