Rather than copy a lot of information into the Forum chat, the procedure can be found in the DP 6.2 Installation and Licensing Guide, Chapter 4
Migrating from 32-bit/64-bit Windows to 64-bit Windows/Windows Server 2008
The Guide is available in the GUI by clicking on Help -> Guides
"After migration I can upgrade the DP6.21 to DP9 with trial license directly?"
Unknown, but, it is my feeling that the 60-day temporary license that you got when you upgraded will be gone
"If my upgradation to DP 9 fails unistall DP install DP 6.21 and move the
Program Files -> Omniback -> db40
Program Files -> Omniback -> Config -> Server
to original location."
Correct