I'm on a Mac and I just opened up some 2012 files with 26. Everything was there, I did get the message asking if I wanted to use the new articulations or the old ones, I chose to use the old ones so any manual positioning I did on this file would be retained.

I don't have 2009 any longer so I can't be sure, but I would suspect that the files would be okay. Download the 26 demo and find out yourself before purchasing it.

I have files much older that still open. 

Normand,

If you are unsure, you could always load the Finale v26 30-day demo and try it out (assuming you now have a 64-bit computer).