First off the good news: The whole React Reform experiences is now much more rounded. Also: existing your <Form>'s don't need to be updated. But probably everything else.
Let's have a look at the details:
Rather than registering themes and validations in some global module, you now need to use ReformContext. Check out the docs for more details.
For creating themes you now have to use the createTheme(themeObj) method. You probably don't have to change too much. You need to replace the old <Fields>[fieldDescription]</Fields> component by children and the fieldDescription now gets its place under the renderField key within the themeObj. The docs explain all the details.
Got a lot more powerful and explicit. There's a simpleInputWrapper that almost looks like the old wrapInput. Have a look at this and the docs to see how to migrate your code
Instead of validators we're now more consistently using validations everywhere. This also means that you need to do import defaultValidations from 'react-reform/opt/validations' to import the default validations.
If you were calling validate() on the <Form>'s reference. You now need to call checkForErrors().