As I understand I should open support request to clarify this question.
McAfee Gold Support has clarified my questions.
I hope it will be helpful for someone else:
1. As I understand from FAQ https://kc.mcafee.com/corporate/index?page=content&id=KB79784I have to use EEA extension 7.0.4 to manage both version. Is it correct?
A: Yes, correct.
2. I should run EE: User Data Upgrade task beforeinstallation of DE 7.1 extension and laptop software upgrade. Is it correct?
A: Yes, this task should be run.
3. I should implement DE 7.1 extension before DE 7.1installation. Is it correct?
A: Yes, that is correct.
4. Can I manage EEPC 7.0.2 clients after DE 7.1 extension installation?
A: Yes, you can manage EEPC 7.0.2 after DE 7.1 extension installed.
I was loking for information about backward compatibility in KBs and FAQs but it was here in community:
I put together a video to show the process of upgrading the extension to 7.1.
Note that the upgrade of clients is not required during the extension upgrade process - and can be done at leisure once the extension is at 7.1.0.
Thank you for Video link.
The upgrade process is more or less clear.
But the backward compatibility topic was not clear for me.
Could you please clarify what is the best way to upgrade 6.2.1 clients basd on next KBs:
As I understand that better use 6.2.1->7.0.2->7.1.0.