one hospital two different unions, one profession

Our hospital recently under went a merger(we took them over). Our union Registered Nurses have one union on two different campus. The merging hospital has a different union at a different location.

Our Unit of 50 people is being sent to the merging campus with a different union. We are being told that we will have to drop our current union and then join theirs. They get to keep all their jobs at that unit, we must bid on ours. Jobs will be lost.

Wondering why we as "One" can not take a vote and vote on which union as a whole we want to represent us. When the merging occured our Head person sign a clause that that union will remain at that campus. can he do that. What can we do?