If it were me, I'd flash the firmware of the controller up to version 19, and then you're all set.
But, if you do want to leave it at v16 and just copy your code across, here's how I would do it:
- Make sure you have RSLogix5000 v16 installed on your computer. As George said, you can have multiple versions installed; when you go to open a file the software looks at the version it was written in and opens that version of RSLogix
- Copy and paste your v19 file to a new location and open it (it will open with v19)
- Right click the CPU in the I/O Configuration tree and click Properties
- Click the "Change Controller" button
- On the dialog that appears, change the firmware revision to 16
From there, the software will do a conversion, which will involve saving a v19 copy as a backup, and creating a new v16 file. The v16 file will be opened automatically. If there are any compatibility issues, you will usually get a dialog box warning you about them.
Once it's opened up, verify the controller and you'll soon see if there are any compatibility issues.
If you find that there are no compatibility issues, great! You're sorted. If there are any significant ones, you may have to weigh up whether it's easier to try and fix them, or to just flash the CPU firmware to v19 and use your original code.
Hope that helps!