Migration in batches vs All Process Migration

Hi,
One of the client has 41 processes and 12 robots to be migrated from 17.5 to 19.0. New infra will be provided for 19.0 migration. There are 3 environments with HA setup in Production environment. Additionally, there should be no downtime in the entire migration. What will be the pros and cons as well as approach on the two scenarios?

Scenario 1: Migration in Batches:

  1. Migration of the processes should be done in three batches of 10 processes , 16 processes , and 15 processes

  2. For batch wise migration, should we use import and export functionality.?

  3. Can 2 environments run in parallel for a certain period?

  4. There should be few robots and processes running on Old Environment and some robots (and processes) running on old environment

  5. What will be the roll back mechanism in case any robots in new environment fails ?

Scenario 2: All Process migration:

  1. Migration of all 41 processes should be done in a sequential manner in 3 environments (DEV/UAT/PROD)

  2. All processes should be migrated in a single shot in each environment

  3. Migration Workbench Utility as mandated by Product Team to be used by team for the activity

  4. When in Prod, we would want to run 2 environments parallel for a week till new environment is stable.

  5. Old environment will be cut over after new environment is up-and-running.

  6. If new environment has issues, we will be required to switch ON the robots on old environment.