Diane has not changed personality, or character. She's one of the few HO characters who haven't changed drastically over the past year.
She has always been an outwardly good, moral person who will happily resort to immoral, illegal actions to get the outcome she would prefer, and damn anyone else - including her own children and fiancee:
1. She had an affair with her teacher. I still laugh at how much of a pious hypocrite she was to Jen, reprimanding her, when she lived in a house payed for by her former teacher who she slept with.
2. Both her children had a child in their teens. Ironically, Bella and Katy are probably the only people other than themselves that Finn and Sinead care(d) about.
3. Diane stole another woman's child so she could become a mother, despite already having two children she had raised. If Diane had focused on looking after S & F instead of wanting her "own" child, maybe they wouldn't have ended up so abhorrent!
4. Diane covered for Sinead when she was revealed to be a vicious bully, and played a part in Esther's suicide attempt by being needlessly cruel and nasty to her. It always makes my blood boil when teacher's treat pupils like Diane did to Esther.
5. And finally, she's covering for Finn over Blessing's attack.
Being nasty to Esther, being pious to Jen, acting superior towards Ste, lining up to hate on JP, letting Blessing suffer, are we seeing a pattern here? All LGBT. So that's where Finn got the subliminally homophobic messages from - Diane fears and dislikes anyone who isn't like herself, or acts differently to her - Frankie, Cindy, Maxine.
She's a true bigot. An utterly loathsome woman. I used to like her character, but she's always been a nasty, selfish person, IMO.