I do not see this happening, star network is doing great job in showcasing Ajay Sir's films and there is mutual admiration, strong bond between them for almost over 10 years. Ajay Sir's films have been constantly doing well and surely Star Network would accordingly increase agreement cost once up for renewal , even if zee offers more i think Ajay Sir will stick with Star Network.
Also had ur information been the future scenario then till now few of Ajay Sir's films would have started to shift to zee like Wanted, Partner, Hello Brother etc moved to zee before their agreement with Salman Ji end. Till now even Singham is being aired on Star Network and even movies older than it whose rights r held with Ajay Sir, Zee only has his movies which r produced by Harry Baweja, Indra Kumar or sone other production firms and thus do not come under Ajay Sir's film deal