Thanks @dadoonet for reply and It really help to clarify many stuff.
So just to reiterate for my confirmation
• Reindex and Upgrade to 5.6.16
•Use the upgrade assistant to check what needs to be done
•Upgrade to 6.8.6
•Use the upgrade assistant to check what needs to be done
•Upgrade to 7.x. I'd recommend 7.5.2 as this is the latest one.
- When we are reindexing using api then I believe the will belong to target or destination elastic search.
- Is it possible to back out the change (back to 5.5 version in case of any issue?
Apologies in case my questions are too basic. I will be doing this for the first time?