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When I look at my offers I see I'm prequalified to upgrade my Delta Gold to Delta Platinum. What I don't see is any sort of bonus offer to make the upgrade. Are there ever incentives to upgrade?
There are. Unfortunately, if you don't see an offer at the moment, it simply means you can still upgrade, but no promotional upgrade SUB is available. There's really no set timeframe when these become available.
As @FinStar said, you won't always get SUB when upgrade appears. SUB can also have potential of increasing, the longer you don't accept one.
I'd hold off until they give you some incentive to upgrade.