I tried to migrate a non dotnet nuke portal to dotnet nuke portal version 4.x,the first problem you will face is that you want to migrate your existing users to the new portal ? this post will show you how to do it in easy steps :
1) Export your users into an excel file.
2) create a script to that call 2 stored procedures in DotNet Nuke DB :
dnn_AddUser (in some versions its called Add_User)
You can create a script for all users exported in excel file using new feature in Office 2007 called Field Merge,simply open new word file and u will find it under mailing ribbon.
you want to know how to create a field merge in word 2007,simply i will invite you to watch my friend's webcast to how to create/use field merge in word 07.
http://dotnetking.com/HowTo.aspx - > WOW....... it's free.
After creating this script in word file,execute the script in any query analyzer on the remote or production server,and u will see all your users has been migrated successfully.
Enjoy ! with Field merge :) thanks Microsoft for this great feature.