Some rostering agreements are contractual but many are not. If your agreement is union negotiated and that union is removed then the company may voluntarily honour it...but why would they want to?
This all depends on the company approach and what they are really up to. I have been lead to believe that VS are plotting a major assault on terms and conditions across the board. That may be just rumour - only those inside can answer for sure but it's no secret the place has been run like a high school project for years. Now that the Americans are heavily involved I would expect major (unwelcome) changes to employment practices.
If you were running the place and somebody gifted you the opportunity to get rid of costly working agreements what would you do? If the company removed an existing Union and replaced it with another voluntarily they would set up a new collective labour agreement. Lots would depend on the scope of this. An agreement can be forced legally if a union pursues statutory recognition however the scope of this only revolves around pay, hours and holidays. Anything this limited is generally poor and seldom worth the paper its printed on but represents a start in the long process of improvement. This would normally be first step in establishing a union presence rather than a process for an already established unionised workforce.
Of course a company might promise plenty to a fledgling union then fail to deliver on it. This may take the form of keeping the framework of agreements in place while striping out the detail that makes them worth anything. The quandary then becomes what can they do? Strike? The idea of any form of industrial action is fantasy unless the union has established a significant strike fund. This requires £several million as they will be injuncted, subject to lawsuits etc etc.
Ironically the best course of action for crew in such circumstances would be for both unions to reach joint agreement. This would enable pre-existing agreements to be kept under a power sharing council. This would not be ideal for either union but their respective negotiating teams should know it is realistically the only way to ensure continuity. How the company respond to such ideas is key. If the company do not entertain that as an option then you can rest assured they are seeking the removal of existing agreements. That is the point where it is time to start being afraid.
competition is generally a good thing..even in unions until the point where both sides take their eye off the ball because they are too busy squabbling. The only winner is the company in these circumstances.