Used regular expressions to convert do to opt whenever possible.

For example,

if\s*\(\s*(.*)\s*!= NULL\) \{\s*\n(\s*)(?:\1\n?\s*= )?FREEdoFreeTree\s*\(\s*\1\);\n\s*\}

to

$1 = FREEoptFreeTree ($1);

And similarly for FREEdoFreeNode and TRAVdo, and for slight variations.

Thanks to Michiel for providing this regex.