Just use the find and replace built into Word. The symbols are just characters in the font that makes the symbol for you for a given character.
So if you need a shadowed checkbox symbol, a lowercase q is inserted as WingDings.
Find the symbol in your document, and highlight it. the Font dropdown will change to the font for the highlit character. Then find out what character it is.
Using Find and replace, specify the font, the character, and match-case. Replace with the font and character that creates the symbol you are after.
In the example you gave, both are in the Symbol font. Theta is upper- or lowercase Q. Pi is upper- or lowercase P.
Symbols are nothing special; they're just characters.