Hi Ganesh,
If you want to upgrade from 6.4 to higher. I would suggest to go with 6.6.1 directly.
(1) Has anyone done such migration? If yes, then which version did you choose to go with? : We chose 6.6 directly
(2) Are both 6.5.1 and 6.6.1 backward compatible? Or, did you have to change your current automations to make it compatible?: No changes required to import 6.4 based automations into 6.6.X version. However, you need to remember that, once you have imported into 6.6, You may not be able to re-import the automation (after export from 6.6) into 6.4.
Hence, if you decide to go with 6.6 , please maintain appropriate backup (just incase you want to revert back to 6.4 later).
(3 ) What has been your experience with 6.5.1 or 6.6.1?: No major difference.
(4) Is there full backward compatibility? : Yes, all automation built on 6.3.x and later can be imported into 6.6. Just remember if you upgrade from 6.5 to 6.6, there may be need of changing the code a little bit to handle queue items as posted in some other posts as well.
Hope it helps.
------------------------------
Vivek Goel
RPA Architect
Asia/Singapore
"If you like this post, please press the "Recommend" Button.
------------------------------