changing metronome bpm without changing the tracks already recorded?

I just had a guy come in a sing a rough accapela version of a doo wop song he wrote and wants music for. He did not use a click but is fairly consistant. I figured I could figure out the tempo from his vocal, set up a click, slide it in time to line up, fix any issues with melodyne... This is just a rough anyway, and then write the music... But when I change the tempo his vocal stretches or shrinks with it. How do I keep it as is? I tried locking the clip but it still changes. Thanks.

How did you do it?

If an audio track doesn't start at time zero and you want it to retain is absolute timing relative to other tracks when changing tempo, you can set its Timebase to Absolute in clip properties. By default audio clip start times follow the M:B:T grid when tempo is changed.
 
Also, you might want to look into using Set Measure/Beat At Now to sync the timeline to freely performed audio or MIDI. Basically, you just tell SONAR which hits need to fall on which measures and beats, and it sets the necessary tempos to align the timeline for you without altering the playback. And then you don't have to worry about the timebase of clips; SM/BAN automatically maintains absolute timing of everything as it fits the timeline around it. You can set as many or as few points as necessary, depending on how wild the timing is.