Blocks produced after recent protocol upgrade have a priority over stale chains.
Desktop users are encouraged to upgrade.
Existing masternode upgrade is not required. New masternodes must be bootstrapped with this version.
Mining pools and exchanges can keep using existing version (v0.8.4.1), though it's recommended to bootstraps new nodes using only recent version (v0.8.4.2).
SHA256 checksums:
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cf165c72c779b35eaea264d38c792ccfe1bdfff98d9371c6ff4830bdcfaf41fa iqcash-gui-linux.tar.gz
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252fc32359c3df6add541d34509ca7842a4fd4b9e0023450cc44a41a2deadb67 iqcash-gui-osx.zip