The Achilles heel of JDE development / ER code (one of a couple).
Short answer is you can't. I rarely ever do this so someone that has done this more than I can probably give you a more detailed/definitive answer about changing the business view on a section or form. However, I believe sometimes if you start by copying the original BV and then add your tables/fields, etc. without removing any existing tables or fields, you can sometimes "slip" it in to a UBE section or form w/o losing too much. You still should probably review/click through all your ER code and any data bound report/form objects to make sure everything is still there. The ability to do this does somewhat depend your release too I think.
Let me step up on my soap box for a moment. This is one of the main reasons why I put as much code as reasonably possible into C BSFNs. You don't lose all your code when you decide to make some little change. I simply use ER code to tie all my BSFN calls together. By doing this when I do change a view, or table or something the impact is as minimal as possible. Do I still do some simple logic in ER code? Sure, but if I am using a UBE section to read in records and then do a bunch of business logic you won't find a big massive block of ER code in the DO event, you will find a BSFN call.