If you don't want to or you can't upgrade due to security policies on your computer, you can download the binary for the newer puttygen.exe, run the binary and load your .ppk file. Once loaded, go into "Conversions" and select "Export OpenSSH key" and save as a .pem extension.
Next, using the older puttygen, go into "Conversions" and import the .pem file you generated. Once loaded, "Save private key" to a new .ppk file.
At this point you should be able to use the older putty.exe file to connect to your host with the .ppk file that you saved.
You can verify, assuming you have the .pub file, by saving the file as a new public key and compare the signatures between the 2 .pub files. The public key signatures, from my experience are the same.
Or as some has already stated, load your .ppk file, go into: Key -> "Parameters for saving keys files ..." and select PPK file version 2 and "Save private key".