@G, Balaji (Cognizant) Thank you for reaching out regarding ASE migrations.
Customers have had success with using the migration tool and I have not personally encountered any issues. I understand wanting to ensure your migration plan is ready for any issues though.
If you are migrating production environments and you're concerned about receiving support during or after the migration, you would want to consider having a technical support plan in place to receive assistance. We have three tiers of support plans. The dev tier is likely not going to be quick enough (8 hours and no weekend availability). For this reason, I would recommend the standard support plan.
You can purchase the support plan and then cancel it once you're done. It's billed on a monthly basis so if you can complete you migration and post migration validation within a month, your costs would be low.
Please see here for more information on the support plans: https://azure.microsoft.com/en-us/support/plans/
If you were to encounter any issues with the migration, you would want to proceed with creating a support case via your support contract. You would want to create the case as a Sev A, which would require a response from the team within 1 hour. They would then assist you with troubleshooting the migration.
We can provide you with basic assistance here on the forums, but we are largely only staffed to provide 5(days)x16(hours per day) support on here.