dear Beatzmidiot- no, not at all- it wouldn't damage the previously created tracks. Try Vari-Speed in Reaper if you don't remember it from the old analog days. Now in a project with un-frozen MIDI tracks, that may be different/more difficult, depending on how well they could implement Vari-Speed in Sonar. I bet they could even implement it without tempo change in the new world order, but- hey- easy for me to say!
But Vari-Speed just raises or lowers the playback speed/pitch of everything while you track a new instrument. Then you can turn it off and everything should be the same as it was.
And panup- sure... it's easy to jump sequentially from one marker to another- one by one- forward or backward... But that's not like having a marker available at one button by it's number, instantly.