Completely new roots are better, but don't rule out new growth on old roots altogether.
What you have to consider is just how different, in terms of moisture-holding, the old medium is the the new.
In the case of an old root with new growth, the new growth will certainly tailor itself to the new environment, but the old part doesn't. If the new conditions are "too much" for the old parts, they can die and take the good parts with them. By contrast - and probably something we have all seen (especially in plants in old sphagnum) - that old root might just become a "static" connection between the new, active root and the plant, and carry on for years.
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