Are there any reasons the replace feature will not work?
The replace option only works on stored Salesforce data. So if you have disabled our triggers or have chosen 'Don't sync members of this list to Salesforce,' the replace will not work. It is worth noting that it is rare to have either in place. A more likely reason is that the primary sync has failed, and therefore data in Salesforce is not up to date. Again this is rare but worth checking.