This is definitely wrong and the chapter should have a talking to about it!!!
It is one thing to adopt a little if her big drops out of school and is never heard from again...usually, someone within the family tries to "take care" of her. That's not always possible, and I can understand an adoption. My 5 adopted a big because her big dropped out. But she dang sure was still in my family and proud of it!!
But to switch after someone graduates, especially if they come back to visit, and not claim the old family at all anymore??? Let alone if they're in school?? That would be so upsetting I can't even imagine.
We had sponsors (ruby sisters are what they're called now, I think) for the first 2-3 weeks of pledging and then picked bigs & littles. This was mutual selection...same as bid matching.
Your big doesn't have to be your bestest friend in the sorority forever, but it is a special relationship and not one you should switch at the drop of a hat.