You will be stuck on an incompatible chain following the old rules and you will be unable to send ether or operate on the post-upgrade Ethereum network.
A network upgrade is a change to the underlying Ethereum protocol, creating new rules to improve the system.
Before Ethereum performs network upgrades on the main network, test networks, such as Ropsten, are upgraded to test the changes.
The original Constantinople changes, listed in this blog post, were applied to test networks before the postponement and require a 2nd network upgrade to reverse the original Constantinople changes. Petersburg and it occurs on the same block number as Constantinople.
The Ethereum network will be undergoing a scheduled upgrade at block number 7,280,000, which is predicted to occur on Thursday, February 28, 2019.
The exact date is subject to change depending on block times between now and then and could be activated 1-2 days before or after.
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