It is a mandatory upgrade. You must be upgraded by block 1,220,000 (roughly 25 days away). Initial release of UFO using the Bitcoin 0.11 codebase.
IMPORTANT - Hard Fork on block 1,220,000: Improvement to the eHRC difficulty.
Version 4 blocks are created with this release which after 75% of the last 1,000 blocks are version 4 then BIP65 OP_CHECKLOCKTIMEVERIFY and BIP66 Strict DER Encoding becomes active. BIP65 is the more exciting of the two and will allow us to join decentralized exchanges using onchain atomic swap.
Other changes: Block file pruning Big endian support Memory usage optimization Disable wallet transaction broadcast Stream isolation for Tor