After switching my installation to sylius beta 1, i noticed that some form classes like this one has been declared final, which makes then non extendable.
In our project we need to give the products admin the ability to set more information from the admin ui form. Before switch to beta we managed to do that extending that ProductVariantType form and overriding the related template. Now it looks ProductVariantType can't be extended anymore, so:
Have a look at the docs (http://docs.sylius.org/en/latest/customization/form.html), we described there best practices for customizing forms.