You would not have to pay $400... the policy is to take 100% of previously purchased extensions included in the integration off the integration price up to 50% off the new extension, so you would likely get 50% off based on your order history.
Also it includes a major update to Data Assist which is well worth the additional cost alone.
Jason misspoke in saying that other extensions definitely won't be updated. Some may be updated if necessary in the upcoming months. The next extension we are going to update will be eCart and then we will evaluate what else needs to be updated and in what order based on user feedback. We do strongly suggest upgrading to Design Extender and Data Bridge since we have around 30 stand-alone extensions it will take some time to test and update them each individually.