You can view history in from Josm.
Select the **relation**, View, History and you can view the changes that were made to the tags of the relation.
There is a dropdown box next to each listed changeset and you can select to view the changes in that particular changeset... I normally view in Achavi which displays the changed ways.
I think this is the way you want to view but am not sure if it shows all the changes.
You can instead **select members** of the relation and view the tag changes on the members. Beware of doing this on a large relation with many members as it can be unmanageable. Sometimes it may be better to select just one or few members to view.