Now we are on 0.14.x software, what would happen if someone opens a 0.11.x, 0.12.x or 0.13.x GUI/CLI?
Also, when Monero forks again later in 2019, what will the 0.14.x GUI/CLI present to the user?
When starting the daemon,
monerod, or one of the command line wallets (
monero-wallet-rpc), if you are running an old version, it prints a warning message telling you an update is needed. It's worth noting that it does this by checking your blockchain height against the hard-coded fork heights, not by calling some remote update server.
I'm not as familiar with the GUI, but presumably it warns you also.