Princess of Change is what I believe she would be if redeemed, it's kind of boringly obvious but so are all the other princesses.
Why would she be demoted to princess though? Also, she does look seasonal in that getup, so change fits that picture at least.
No one is calling Cadence an Empress even though she is one, and Chrissy would no longer be full of herself if redeemed and would more than likely take that title as a sign of good faith.
Pretty much, Celestia has clearly been a Queen for a thousand years, but she's called a Princess. Luna should technically be an Adviser or a Consular since she has a thousand years worth of government and society to study up on before she should be considered a ruler, but she's immediately a Princess.
Yeah but, it doesn't work like that... If you're a Queen you stay a Queen, you can't get demoted to Princess... and Luna being banished for a thousand years doesn't change the fact that she's a Princess. O__o
Not to mention that they throw around the 'alicorn princess' title as if neither word has an independent meaning.