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Yikes! I assume you are doing an explicit example and now want to generalize.

Sorry, I think you will have to walk the expression tree. Hopefully I am wrong, but Sage just doesn't have methods for this built in yet. I don't even know that Maxima does. Axiom/Fricas might be more along those lines, I don't know. Honestly, I'd probably even do a string thing, as long as I could watch it step by step - like in a text editor - to make sure I wasn't making any erroneous replacements.