Make certain your main "page" subform Flow is set to positioned to start with.
Now wrap your content in subforms.
You may have several subforms from doing this. For instance, let's say I have a header section where I'm asking for a persons name, address and phone number and I have all of these fields lined up side by side. Then I have a large comments field underneath which I want to expand. Under that, I have more smaller fields and then another expanding field. Based on that type of form, I'm going to make 4 subforms. I'm going to wrap my header fields up to the comment field in a subform. Put the comment field in its own separate subform. Wrap the additional items in another subform. And finally, wrap the last comment field in a subform.
Now to set the flow. Leave the subforms with lots of non-expanding items in them as Positioned. The subform that has your expanding comments field, set to flowed (and allow page breaks, if that's what you want). Do the same for any other subforms that need to expand. One you have everything within the page wrapped in subforms and the flow of that set correctly, THEN set the flow of your "page" to flowed. Your content should still look the same, and you should have non-overlapping expanding fields.
