Consider building another layer of abstraction into your current code.

As I understand your question, you don't want to get stuck searching-and-replacing function names in the code that uses these two libraries. By and large, that means both your brain and your survival instinct are in good working order. ;-)

A set of wrapper functions will let you modify your client code once, then switch between the two different libraries with ease. If your client code presently looks like this:

    {...}

    $some_value = old_parser_function_X (@args);
    
    {...}
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Swap in a generic function name, and then create a new function which calls the original one:

    -- client code --
    {...}
    
    $some_value = generic_function_X (@args);
    
    {...}
    
    -- wrapper library --
    
    sub generic_function_X {
        return (old_parser_function_X (@_));
    }
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The payoff is that you can now write another wrapper that calls functions from your new parser library:

    -- new wrapper --
    
    sub generic_function_X {
        {assemble a result by calling:
            new_parser_function_A (@some_args)
            new_parser_function_D (@some_other_args)
            new_parser_function_N (@still_more_args)
        }
        return ($result);
    }
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and it will work as a drop-in replacement for the old wrapper, even though the two parser libraries have completely different APIs.

Writing a test suite that compares the two wrapper libraries then becomes a one-banana problem.