Does the migration of R75 or R77 are easy or not ? or not possible ? risky ?
From R75.40+ versions, I believe it is a single-step process, assuming you're using Gaia OS.
If you are coming from earlier versions (or not using Gaia, which will be the case pre-R75.40), then it will be a multi-step process.
Check the Upgrade Wizard with your exact versions to get an idea of the version upgrades that will be required.
What I would do is "test" the upgrade using the Migrate Export/Import process.
I describe the process here: TechTalk: Migrate to R80.10 and New Years Toast
Retrieving data ...