When there is a new mainnet version, does it mean that there is a needed upgrade of Kiln node? If so, how can we upgrade? Does it mean just downloading and installing without uninstalling the old version? Is the baker safe during this procedure?
We (Obsidian Systems) update the version of Tezos that Kiln uses when we do new Kiln releases. When you upgrade Kiln, your node will momentarily be offline. When Kiln restarts, your node will start up again.
Best time for your upgrades is when you do not have opportunities to bake or endorse a block since they will be missed when your node is down.