So  my friend sent me a file with a synth track with a sound from Kontakt2. It won't play, I don't have Kontakt2, I have Kontakt 5. How can I get the track to play? Can I change the synth somehow?
 
Thanks.
 
Rick Neglia, new at all of this.

Welcome to the forum!
The quickest way to change the synth is insert Kontakt5 and drag the MIDI to the respective MIDI track.
What do you mean "it won't play?" The Now marker doesn't move? It moves but no sound?? 
Is it an instrument that is included in Kontakt 5? If not, you have to get it and add it in your Kontakt library.
If the instrument is in K5, then maybe the path to the library is messed? I don't know if the path info moves with project or if it's defined by host SONAR. I'm not even sure how well backwards compatible Kontakt is, as they tend to make older instruments useless with new versions. EDIT: I mean the other way round - instruments made for Kontakt5 don't work in the older ones.
It may be you need to replace the instrument with another, I'm not sure.

Have your friend freeze or bounce the synth track. Unless there is an understanding about which plug-ins are available on both installations, it is easier to share audio only projects.

Thanks for the replies. Got it now.