How to update conflict resolver when upgrading from SQL-Server 2005 to SQL-Server 2008
Well .. so nobody got an answer. But I think I (finally) got it. Guess what... it is somewhere in the metamodel (as usual)!


When adding an item to the subscription, the new conflict resolver references to be used by the stored procedure come from the [distribution].[MSmerge_articleresolver] table
But, for existing subscriptions, previous conflict resolver references are stored in the system tables of the publishing database, ie [sysmergearticles], [sysmergeextendedarticlesview], and [sysmergepartitioninfoview]


So we have on one side an item initialy published with SQLSERVER 2005, where the publication references the 2005 conflict resolver, as per the publishing database metamodel. On the other side, the machine will attempt to add the same item to a new publication, this time with a default reference to the conflict resolver available in the distibution database, which is indeed different from the 2005 one ....  

To illustrate this, you can check the following

USE distribution
go
SELECT article_resolver, resolver_clsid
  FROM [MSmerge_articleresolver] WHERE article_resolver like '%Later Wins%'
  GO


Then, 

USE myPublicationDatabase
go
SELECT article_resolver, resolver_clsid
  FROM [sysmergearticles] WHERE article_resolver like '%Later Wins%'
  GO
 SELECT article_resolver, resolver_clsid
  FROM [sysmergeextendedarticlesview] WHERE article_resolver like '%Later Wins%'
  GO
 SELECT article_resolver, resolver_clsid
  FROM [sysmergepartitioninfoview] WHERE article_resolver like '%Later Wins%'
  GO  


So it seems that I should update either the references in the distribution database or the references in the publication database. Let's give it  a try!