The node itself does not change, but the context in which they are spending in does. Thus, if RIGHT NOW you were to create a transaction with a segwit output, it is an anyonecanspend output and anyone can spend from that output
So the security is to make sure this output can never be spent by old nodes, how can you be sure of that?
Suppose two pools with 60% hash rate, and they are sitting on the side line watching the mined blocks, once 35% of blocks are mined as segwit support, they switch to mine segwit blocks and activate it, then there will be many segwit transactions out there, and then they switch back to original software
Now network are full of anyonecanspend transactions that is free for them to grab, and they have 60% of hash power so their chain become the longest, since majority of the nodes are still old style nodes (as in soft fork you don't need nodes to upgrade), these nodes will build on their chain, and the rest of the segwit nodes will fork into a minority chain since now they reject old style blocks
So it is a mess after this accident, all the people who used segwit transaction will lose their money and they don't even understand what happened
Nope. It's all in the deployment. There is a lock in period (grace period) before the rules go in effect. See BIP 9:
for how the deployment actually works because they will be using versionbits.