Here's my prblem:

1- you are creating new Pipeline
2- Copying deals from old Pipeline to the new one.
3- Now you are trying to adjust the time when those new deals moved between stages? (The stages are not the same than in previous one)

Unfortunately there is no way to change those dates as they are getting registered through the system and are there by default.

It would be a useful feature to adjust days between Pipeline stages afterwards? We could also calculate the time conversion / stats for our old customers.