Monero’s future software upgrade is mostly a hard fork but without creating a new coin. The implementation was done to make it functional for hardware devices or other CryptoNight coins/forks. Keep in mind that before the network upgrade, all CPU and GPU miner will have to update the mining software.
According to the official statement on Reddit:
“It is highly recommended to make transactions only with an own full node/after the blockchain sync.”
It’s also stated that these precautions were made for privacy reasons.
Here (https://www.reddit.com/r/CryptoCurrency/comments/82w91m/monero_network_upgrade_at_block_153950028th_of/) you can find an official Monero statement about the upcoming XMR upgrade.
Update. Hard fork postponed to the 6th of April 2018 and should happen on block #1546500. See related post.