For athletes who switch coaches and programs, what is some advice in terms of the transition?? Further situations would be athletes who leave their coach mid-season. What would be advised of the new coach?
Contact the old coach as a start.
Discuss with the athlete(and parents if appl) why they are leaving a training program mid season
From a training standpoint, the goal must be to minimize adaptation stiffness while successfully ensuring the the athlete adjusts to the new plan.
In order to accomplish this, as has been referenced, the coach must gain as much information as possible in regards to what the previous training consisted of as well as determine the athletes preparation level, tolerance to varied regimes of stress, likes/dislikes, how much they know about their own response to training and restorative measures, injury history, and so on.
Find out their previous training regime and make very subtle changes, if any, initially. Most of the athletes that have come to me have had less than optimal training programs previously. If they are starting out at the beginning of a new season, I have them integrate into our program/group pretty seamlessly - as we spend a lot of our general prep working on technical issues related to sprint mechanics, proper warm-up and lifting technique. And, we have broad progressions that allow them to adapt appropriately.
I’ve had other athletes join in mid-season, and we’ve been very careful about simply having them “jump-in” to our training program. The issue of short-to-long vs. long-to-short must be determined right away. Going from a L-to-S program to a S-to-L program can be tricky. The L-to-S athletes are not ready for the high velocities and intense acceleration requirements of a S-to-L program, so don’t have them do the same type of work. There must be a dovetailing of work over a given period of time. Otherwise, the stress of a S-to-L program will result in significant problems related to mechanics, overuse injury, soft-tissue injuries and CNS fatigue.
what if they have a plan that is unstructured yet have some targets that based on previous performances could be achieved? 
If they can achieve their targets with an unstructured training plan, 1) the plan is not that bad and you need or 2) the athletes are young and still have good progression margin and would improve anyway, but for how long and with which injuries, that’s the question.