Picking the right actress for the job is indeed an art.
As you pointed out, Yael was going for Nicky but was brought back for Lorna.
We've seen in many interviews that Laura was trying out for Piper "even though the actress knew she was wrong for the part... and brought back as Alex.
Uzo said she tried out for 1 role (did she say which??) but was brought back for "sweet" Suzanne/Crazy Eyes.
IIRC Lea was trying out for the role of a guard and came back as Big Boo.
The other thing that has struck me over many interviews, was that the actress was often told she'd be there for a limited role 1-2 eps, but something caught TPTB's imagination and the roles blossomed (Lorna Morello & Taystee are 2 that come to mind.)
I think the act of upgrading several cast members to "regular" status is basically job security. I don't think it means 1 character will overwhelm the story line, but I suspect its a way to wrap up the actress for a defined period of time.
Oh what fun! A tribute to Poussey http://www.buzzfeed....range-is-the-ne