An encryption subkey can always be used to decrypt data, even if it is expired. So you can create a new encryption subkey, and give your (updated) public key out, and people will encrypt data to your new encryption subkey, but you can continue to decrypt old, existing data with your expired subkey. You will have to send out an updated version of your key even if you choose to update the expiration time, since otherwise your partners will have the "old" version that shows a soon-to-expire subkey, and they will not be able to encrypt to it (once the expiration date comes).
You do not export just the public portion of the subkey. You export your entire public key, and that will include the public portions of your subkeys.
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