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Titanic (1997) has been upgraded to 4K on iTunes with Dolby Vision and Dolby Atmos audio
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If you had it purchased through iTunes in the past, it should be upgraded for free.
Looks like you don’t have it purchased through iTunes, as there is the Disney+ badge right above the Play button. If you have it purchased, it wouldn’t say to open in Disney+. Try scrolling down on that page to see the purchase options