No. That's the way varfun is designed and works to keep from overwriting the original variable with the functional output having operated on the input variable(s). A table cannot have two columns with the same variable name and undoubtedly in most cases not overwriting the original variable would be the desired action--and certainly not being able to identify a new variable created from the original wouldn't be agoodthing™ when the new variable is something far different than the original.
I suppose it wouldn't harm anything to submit an enhancement request to have a way to cause that action, but I have my doubts TMW would see it as enough of a need as to have much likelihood at all of implementation.
That said, if you don't want the original variable any longer, you can delete it and then rename the functional column to that name; that can be done programmatically as the variables names are available from the xxx.Properties.VariableNames struct.
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