I've been using Things for iPhone and Mac.
But, I just switched to an Android. How can I move all of my stuff to an Android GTD app?
I don't really care which Android GTD app.
I have migrated my GTD system from one implementation to another multiple times. I started with a Palm, moved to Outlook and Palm, then Outlook, then several different Wiki based implementations, then to Remember The Milk in several different shapes, then Evernote in two different ways, and finally to the mixed tool system that I'm using now. In all cases, the steps are pretty much the same.
Step 1 is to decide what your initial GTD implementation will look like on Android. Expect that whatever you choose, you will be making changes to it over time.
For one example, my current GTD system is pretty much platform independent, and I work with it from multiple platforms:
Step 2 is to migrate your data from your existing implementation to the new one. I have found this it worked best for me in this order:
With each step, take the opportunity to prune and delete things that you don't need any more. To that end, I have never cared about export/import, I prefer to cut and paste from one system to the other or retype, just to make me look at each item more carefully.
The last time I did this, I spread it out over several weeks. It was something of a pain to be running dual systems, but it let me get a good weekly review after moving all the project data, and again after moving action lists and again after moving Someday/Maybe. When it was done, I felt like I had a really good handle on everything in the system, and that nothing had been dropped or missed.
I should probably note that this has been a 10 year odyssey, and that most of the migrations were in the first 5 years. I think I have been working my current system for 3 years without any major change.