I take Jarrett's rule to a specific metric - if over half of the original track is modified, then it counts as a new credit.
Reprofiling of one element or a few turns hardly carries merit for a new credit - there needs to be a fundamental difference in the ride experience in order for it to count as a new one.
For instance, Phantom's Revenge original conversion from Steel Phantom still carried the original first two drops and lift hill. While original track remained, majority of the original layout had been altered.
In terms of RMC retrack work - new track would most imaginatively carry new element addition, turn reprofile, etc. The sheer cost of simple track replacement would be majority of the project cost - so it would make sense to throw in changes to the layout as well, which would be a minimal expense. New track lay and new layout profiles would merit a new credit. Just new track lay would push the bounds, but I do not think any amusement park would plan to take this route, based on the simple costs of the project.