Upgrading IOS on Stacks

Hey Team,

I've got the job to upgrade IOS on a number of stacks for a client of ours. My question is this:

Once I start the upgrade on the master switch, what will happen to the other switches in the stack and more specifically what will happen to connections on the other switches in the stack? Does the IOS update take out the entire stack until all are upgraded to the same IOS or does it do them one by one?

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I will be using the .tar not the .bin so I can do it all in one command.

I have tried to google this but not found much and I can't practice as we do not have any stacking switches in the lab.

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Thank you.