Hi David,
[quote user="David Larson"]passing null just added a new element.[/quote]
Hmm ... sound weird a bit to me, but probably have not enough information.
From MSTNPlatformNET documentation (SDK Update 4):
If toReplace is null or equal to this , the current state of the element replaces the persistent state in the model. If toReplace is not currently a persistent element, then ReplaceInModel has the same effect as calling AddInModel. If toReplace is from a different model, ReplaceInModel does whatever is necessary to use this element in the target model. If the toReplace Element is not this or null , that Element becomes a nonpersistent element.
I thought the element is persistent (exists already in a model), so null should rewrite the element.
With regards,
Jan