Great question!
Here is a link to our documentation regarding how to migrate DDP's:
http://help.nintex.com/en-US/docgen/docservices/#zz-TaggedUpdates/LOOP%20Document%20Services/getStarted/Migrate%20DDPs.h…
Here is a link to a video tutorial as well:
Migrate DDPs - YouTube
Here are a few things to keep in mind prior to migrating as well:
1) Be sure that Connected Apps is Disabled in both orgs (DDP Admin Tab | Configuration | Settings)
2) Confirm that the 2 following IP Ranges are in your Network Settings for both orgs (Setup | Network Settings)
a. 76.12.32.210 to 76.12.32.222
b. 204.12.67.172 to 204.12.67.172
3) Make sure the Target Org User has been given the DDP Admin Permission set
4) Make sure the "Lock sessions to the IP address from which they originated" and "Lock sessions to the domain in which they were first used" checkboxes are unchecked (Setup | Session Settings)
5) Make sure that the user in both the source and target orgs does NOT have any "Single Sign-On" permissions enabled.