Swift is backwards compatible, so the risk is minimal. As long as the Xcode version you're using is still allowed for builds uploaded to the store you can proceed.
As KMT said, binaries built with older Swift versions are, in general, compatible with new OS releases. And, at a purely technical level, there are lots of good reasons not to update your tools just before shipping.
However, there is one gotcha here, namely App Review requirements. Every now and again App Review updates its requirements as to the minimum SDK you can use for new submissions. As the SDK is tied to the Xcode version, and the Xcode version is tied to the Swift version, you may need to update your Swift code accordingly.
IMPORTANT App Review has just published such a requirement. See Build Your Apps for iOS 11 and iPhone X for details.
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Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
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