The UVic Writer's Guide
When you are talking about an exchange, use replace if the object of the sentence is the old item:
"I replaced my old Edsel with a Porsche."
If the object is the new item, use substitute:
"I substituted a Porsche for my old Edsel."
Dictionary of Usage
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Copyright, The Department of English, University of Victoria,
This page updated September 22, 1995