Using a hard fork (as mentioned in ETFbitcoin's brackets).
So it's not impossible, but very hard and risky, depending on the size and activity of the network (both from a technical and a social point of view, ie. the larger the community the harder it is to consent on upgrades via hard fork).
Can anyone hard fork the network? Or you need certain requirements to do it?
Anyone can. Which is why we got this clusterfuck of Bitcoin hard forks after Bitcoin Cash caused a Bitcoin hard fork hype:
https://bitcointalk.org/index.php?topic=2597083.0Ethereum Classic also forked off Ethereum without requiring any "permissions" -- that's the beauty of an open system like cryptocurrencies.
(sidenote: Technically Ethereum forked off, with Ethereum Classic following the original blockchain while Ethereum reverted the transactions caused by the DAO attack)
Whether your hard fork would be successful is a different question however.