|Try this and see if it works:|
First, make sure that BOTH the new symbol and the old symbol are available in your document.
With the Find And Replace dialog box open
Select the Replace tab.
Now, select an instance of the symbol you want to find and press CTRL+C.
Next, put your cursor in the Find What: text box and press CTRL+V.
Your doing a simple Copy & Paste of the Find symbol.
Now, select an instance of the symbol you want to use as a replacement and press CTRL+C.
Put your cursor in the Replace With: text box and press CTRL+V.
Again, your doing a simple Copy & Paste of the replace symbol.
Click the Replace All button.