Can someone through some light , in case we get 5-6hrs downtime for this change , then what method should we choose for this Alter operation?
We can't really answer that. Only you know what resources you have, what risk/reward you are willing to handle, and how long things may take. For that latter item, your best bet is to try this out on the same/similar hardware and see how long it takes. Do a smaller table and extrapolate if you need to. Or promote one of your replicas offline and modify that. I've given you a low-risk / medium-reward option with check constraints, but for the ALTER TABLE options you really need to try it and see (on non-prod).
What about if the OP opt a strategy something as below,
1) Detaching the partitions 2)Altering individual partitions with required column type and length 3)Altering the table 4)Attaching the partitions back to the main table
This should be faster and also a controlled fashion for each partition individually.
