What is the "middle"? Middle of training or middle of the 1500hrs to go in unfreezing it? If it is the latter then that is wrong, there are and have been many cases where people got the MPL license converted before reaching the 1500hrs. If you mean half way through training, well the ground school is the exact same at least so if anything happens during that you can still change, I know a person who changed even during his flying phase from MPL to ATPL.
This MPL license is made out to be a big bad evil license that traps people when the reality and facts from what is really happening out there show the opposite.