I don't think, that it is possible to change the values of the dropdown by changing only the xml-file.
I found another solution:
1. open the form in Acrobat Pro
2. select in the menu bar Form->Manage Form Data->Import Data
3. choose your current xml-file
4. save the form
Now the values are available.
To change the values, you have to repeat these 4 steps with a new xml-file.