anyone can download a copy of a node and then with a friend. set the 2 nodes to only talk to each other and then change the rules as they see fit.
this is called making an altcoin.

many people have done this and no one cares because its just an alt.

however if you want the network to follow you. you need the majority of the networks consent.
you can set the announcement for 1 week, 1 year 5 years etc(whatever). but if your changes/desires do not meet the desires of the others on the network. they will not follow you.

the current bitcoin debates are trying to get network majority consent or they simply wont bother.
yes segwit may want to split off and ban ~75% of the pools using UASF even if the 'vote' is only around ~25% by this october.

but they then become a minority altcoin. which is less secure and can be attacked by the majority. where by it takes longer for blocks to be solved. the blocks are spammed and many blocks get orphaned.

this is why both sides are hoping for a majority to reduce risks of orphans, hash attacks, spamming and sybil attacks

No there would be an announcement I would say a week or two would be the warning period.
 Could go as long as a few month if people are in agreement to that path but an arranged fork would probably be given due warning before it is done.
Otherwise users would just ignore that fork and continue mining on the main chain so it will not be done lightly.

It's similar to changing your node to do something other nodes will not do that is a fork if no one else follows you then its just a path down to blockchain suicide. Either way signalling comes to mind we will see some signalling towards it first that act's as the advanced notice.

It would be cool if they mass produced usb flash drives with an older version of Core so everyone could plug and play.

i have my own. with a few tweaks.
infact i have core, my own(wrote in different language) bu, classic, bitcoin j and others. i play with them all depending on what im doing/testing

but in short.. if segwit activates.. BU, xt, classic, and al the dozen others 'brands' (if not biasedly banned by blockstreamers) will still get the filtered (trimmed) blocks. but obviously by not having segwit validation. will only be in the hodgepodge of second tier(downstream) part of the network but not fully validating segwit data.. and just relaying native tx's upto segwit upstream nodes..
essntially making BU classic, xt and older nodes just lite wallets.

but if BU, activates.
xt, classic and a dozen other 'brands' will all be ok ALL on same level.(no 2 tier)
 but then core end up seeing orphans and banning nodes to ignore the orphan drama. thus stalling out core, forcing them to either
tweak code to join the majority (give in and raise the block limit).
ban nodes and only white list core nodes to start their minority coin
stay stalled and turn node off

if classic activate
BU xt and a dozen other 'brands' will all be ok ALL on same level.(no 2 tier)
but then core end up seeing blocks it doesnt like .. so orphans and banning nodes to ignore the orphan drama. thus stalling out core, forcing them to either
tweak code to join the majority (give in and raise the block limit).
ban nodes and only white list core nodes to start their minority coin
stay stalled and turn node off

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i think the simple solution is for core to:
keep the TXSIGOPLIMIT at 4000.. that way no matter what the blocksize is, malicious TX's dont start making bigger sigops (EG cores 0.14 16,000 rather than 4000 - facepalm)
and core to put in a RPC and GUI options tab where users can reset the blocklimit live...WITHOUT needing devs to give in/spoonfeed.  without needing to recompile.. so that users are more incontrol and adaptable to change without needing devs to decide for them if/when/how.
that way users can change settings at their leisure and become more independent

Nope, every fork will run through the consensus. because consensus is a rule.

So, don't think the hardfork can be done for the instantly. I don't think if it is possible to run an instant fork without doing a consensus.

i hope bitcoin never fork i like it how it is right now

Perhaps you are right, but only if everything remains as it is, then this is a big risk. How did you say suicide? Perhaps this is really so. But until now it was normal. Maybe I'm wrong.