I really hope someone can help you out!! I'd personally stick with your current person/school of thought for Trials (since they are so close), then maybe switch for your HSC! You'll have to be super self motivated to do it though, not an easy decision to make, only you yourself know whether you have the time and discipline to learn about a significant person from scratch good enough for the HSC.
I know I am in a rather tough situation at the moment, but I could easily change to Sufism (although this is a school of thought and would thus bring up the question of how a school of thought is different to a significant person as I have never done a school of thought before). Considering this logic:
I have to do 2 Section II and 1 Section III.
If the section III question is on linking more than one, i have my other two depth studies to fall back on. If the Section II asks for sig person/school of thought and ethics (for example) than I could do another depth study there, use my minimal knowledge on Rabi'a (which is mostly about Sufism and Mysticism) and lose maybe 2-4 marks, depending on the questions mark allocation.
Besides that, I cannot see any other circumstances that would require me to know about the school of thought/significant person, so should I change to Sufism (a minor change which has some notes) or A'isha (a major change which has great summary notes) or keep Rabi'a (no change but essentially no notes)
Thanks for your advice and feel free to ask me what any of the above means seeing as I think I just had a massive waffling vent over nothing