You don't have to change anything. The length remains the same. The chapter points remain the same.
I'm assuming you're doing this for DVD. I'm also assuming you're using something like Muxman for authoring (and takes frame numbers as input). You're substituting soft telecine for hard telecine. Either way the 29.97fps frame numbers for the chapters are the same. You only have to change the chapters if you do a framecount/framerate change (PAL to NTSC, for example).
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