In these lines from "The Last Ride Together," the speaker is supposing what Heaven might be like. After being rejected by his mistress, he spends the first 8 stanzas dealing with that rejection and trying to look for some optimistic perspective. Although he has been rejected, he decides to enjoy this last ride while he can. He first thinks that Heaven will be a let down after his relationship with this woman ("Earth being so good, would Heaven seem best?"). 

Then he concludes that Heaven might actually be more wonderful than any earthly experience. He thinks Heaven might be similar to his experience of that blissful, loving relationship he recalls with his mistress. When he says, "changed not in kind," he means that it will be as if he and she will be together in Heaven: the same "kind" of experience. Changed in degree means it will be more wonderful, more intense, a higher degree. The "instant" could be taken a few ways. One is the idea that if he died right there, Heaven would be a continuation of "the last ride." Or, the instant itself, his time with his mistress, is relatively short compared to an eternity in Heaven. Therefore, it is relatively comparable to an instant. His elation in Heaven will be more intense and will last forever.