SQL Server - How is a row updated under the hood? When SQL Server updates a row in a table, is the update always done 'in place', or is the row deleted first and then re-inserted? If the update is done 'in place', is this always the case, or are there situations where a delete is done first and then an update?

We have a developer on the team that is saying that when a table has a unique constraint on it, and it's included in replication (transactional replication), a delete is always done first and then an insert. I wanted to verify if this is ever the case. 

SQL Server 2008 R2
I'm not a replication guru, but record may be marked as deleted only in case when an update is generating Page Split.

For the case of replication you can test it by looking at DBCC PAGE and look through transaction log.