Hi,

Nintex classic forms gets migrated as is to Nintex Forms for O365, and you can continue there as is without making any changes, just make sure couple of changes:

• Update the javascript file references if referred using full path (http://xxxxxxxx) as the url gets changed after migrating to SharePoint online. 



• If there is any "Person or Group" control on the form points to context variable,"Current User", replace this with "Current User (Display Name" after migration, otherwise Ninex form loading fails with error "Input string was not in a correct format".



• userProfileLookup function on the Nintex form will not function after migation, to fix this, go to  "Site Contents" page, then click on "Nintex Forms for Office 365" app, then click on "Allow Application Access to User Profiles" link. You will be redirected to a page, asking to "Trust" the formso365.nintex.com app to access user profile service, click on "Trust", then UserProfileLookup function on the form works (this step needs to perform on SP Online site). Make sure you have "Full Control" access on the site to perform this action.



• Nintex form on Calendar list does not support in Nintex for O365, delete the Nintex from after migration and reset the list with out of the box look on SP Online.

These changes are due to architectural difference between SP On-Prem and SP Online, hope this helps.