These modifications don't seem like difficult to do in 4 months, right ?
Making the modifications is not enough. You need to upgrade every bitcoin node in the world as well.
Not really. If you keep your old node running, you copy the the old fork, if sufficient miners go with the old fork to still make some blocks. If you download the new node, you copy the new fork.
And lose bitcoin. How well do you think that will be welcomed by the greater community, and how much would you trust a currency whish did that?
You never lose bitcoins because you run a non-mining node.
This is wrong in so many ways, you obviously have no clue.
Of course not. Whatever happens on a non-mining node doesn't alter anything on a block chain (apart from sending goofed transactions eventually).
Secondly, there are several ways of losing coins due to a fork. Just see the mess that occured when Ethereum split in ETC and ETH. A chain fork can even be designed to steal coins or reverse transactions, like it was in the Ethereum case.
Forking happens by miners. And as long as the original chain is one of the prongs, your coins exist of course on the original chain. You are perfectly right that the ETH fork (by miners !) was done to reverse certain actions by one participant (the "dao hacker"), but he kept his holdings on the original (ETC) chain.
Note that the forking is done by people building block chains and in a PoW system, these are mining nodes. Non-mining nodes cannot alter the block chain, and hence cannot alter any protocol, or any block chain contents ; as such, they cannot "lose coins". If by running a certain non-mining node, you've "lost coins", you can easily get them back: erase your node, and start another one with the "right" protocol (the one that can read the chain that has your coins out there).