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Converting a number back to a symbol?

• To: mathgroup at smc.vnet.net
• Subject: [mg82881] Converting a number back to a symbol?
• From: AES <siegman at stanford.edu>
• Date: Fri, 2 Nov 2007 03:29:06 -0500 (EST)
• Organization: Stanford University

If we have an expression containing integer powers of the symbol u0 and 
we know that u0's numerical value is (or at least eventually will be) u0 
= Pi/2, we know what happens if we write

   expr /. {u0 -> Pi/2}

But what about the reverse case: we have an expression containing 
various integer powers of Pi, and we want to replace all the explicitly 
appearing Pi's by 2 u0.  Writing

   expr /. {Pi-> 2 u0}

doesn't seem to do it.

Is there a way?