I've managed to generate a DFID IATI 2.01 file, and it was relatively easy to do. On the SQL-to-IATI database tool I mainly needed to adapt the view that generates the XML from the core tables. About half a day's work.
I started by putting the elements in approximate order, according to the http://dev.iatistandard.org/201/activity-standard/iati-activities/iati-activity/, and then used the validator to test each iteration on one activity. In our case I started with a hierarchy=1 activity, proved that, and then a hierarchy=2 activity, and finally an entire country file. The validator was fantastically useful.
I'd be interested to know others' experiences.
For us the more challenging thing is to adapt DevTracker data import to 2.01, particularly now everything has moved to codelists!