Currently the (undocumented!) behaviour on this is -

---
    Paragraphs can be joined by user action of delete at the end of a paragraph
or backspace at the begining of an un-indented paragraph. Paragraph formating
style from the paragraph with editing focus will be applied to the joined
paragraph. (Delete retains formating of first paragraph, backspace retains
formating of second paragraph. Backspace on begining of an indented paragraph
first removes the indent as defined in "Bullets and Numberings part 1, 7.4")
---

This function will always delete tab data from the paragraph. Even if the user
wanted to alter the prior paragraphs style to the next one, they can not do so
fully, because hitting backspace at the start of a paragraph destroys any tab
setting. 

Idealy, I would modify the behaviour of 7.4 to restore the indent if a second
backspace occurs. This would allow the user to backspace twice to join
paragraphs without deleting the indent. Then if they further wanted to remove
the indent, they could move to the begining of the paragraph and backspace.