### Steps to reproduce

1. Create a WP of a type which is not a Milestone

2. Make sure, that the WP has duration larger than 1

3. Make sure, that the WP is automatically Begin of the insertionscheduled

1. addEnd of the insertion Begin of the deletion scheduled, eg. by addingEnd of the deletion a predecessor Begin of the insertion(with dates set)

2. and add a child (with dates set)

End of the insertion Begin of the deletion

End of the deletion 4. Try to change the type of the WP to a Milestone type


### What is the buggy behavior?

* There is an error, because the finish date can't be changed (as it is automatically scheduled) but a Milestone needs to have the finish date the same as the start date. 


Begin of the insertion<img class="image_resized op-uc-image op-uc-image_inline" style="width:582px;" src="/api/v3/attachments/389769/content">

<img class="op-uc-image op-uc-image_inline" src="/api/v3/attachments/389773/content">

End of the insertion ### What is the expected behavior?

* Changing the type should be possible and the dates should adapt


###

### Environment information

**OpenProject version**

_15.4_

**Browser**

* [x] Chrome
* [ ] Firefox
* [ ] Safari
* [ ] Mobile Safari
* [ ] Other (please specify)

**Operating System**

* [ ] Windows
* [x] Mac OS X
* [ ] Mobile iOS
* [ ] Mobile Android
* [ ] Linux (please specify distro)
* [ ] Chrome OS
* [ ] Other (please specify)

**Language**

German