Designer compare tools error

Hi every one, I've been traying to compare two codes developed with Logix Designer V21 and V24.
I have both version on my machine, but I get back "Required version of Logix Designer is not installed in this machine".
If you have an idea of what is going up with this, I will appreciate your response. Thank you very much.

you can try open v21.acd with logix24, this will force the conversion and after that do the comparison?

BachPhi, thank you for your response, but I can't follow it. Could you elaborate it a little bit? Thank you.