You cannot lose data in a detach/attach. SQL Server supports the ACID requirements for relational databases. D stands for durability, meaning once a transaction has committed, the changes that transaction made are durable and will not be lost.
Sure, take a backup in case something goes wrong and tell the users that the app won't be available for the period, but that's just good common sense.
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: Discussions on DB performance with occasional diversions into recoverability
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