will M2 site get down or not

Hi there,

will magento 2 site get down during the process of migration or not or site will be marked as maintenance during the time of maintenance

If you do not enable Keep Orginal IDs in all allowed steps of our module, you can proceed with data migration on your live M2 instance.
If you need to enable Keep Original IDs in all allowed steps, you have to set maintenance mode in your M2 instance before starting the migration. Otherwise, it might lead to IDs conflict issues during the migrating time.
Anyway, it would be best if you can set maintenance in your M2. After you complete the migration, you should verify all migrated data ready for use, then you continue with other tasks before pushing your site live.
Regards,
Ubertheme team