The node self updates. The client/baker/endorser/accuser don't.
In order to participate to the chain after that the protocol changes (if it changes) to Athen, you'll have to download/compile/install by yourselves the "client" binaries for this protocol! They are independent from "clients" for the previous protocols.
The _004_Pt24m4xi softwares (as well as the updated tezos-client able to "speak" Pt24m4xi protocol) are already available today in the repository and the docker images. To avoid interruption of baking activities, you have to have started the _004_Pt24m4xi baker/endorser/accuser daemon in advance of the switch. You can do it at anytime, they will quietly wait for the node to switch to Pt24m4xi protocol. Similarly, _003_PsddFKi3 daemons will quietly stop participating as soon as the protocol changes; No need to be there to kill them...
The tezos-node software carries a smarter mechanism because it can automatically continue to validate blocks without any "manual" update even if the protocol changes!
Tezos protocol comes with self amendment. That means that, under some given rules (the one of a vote), a valid block of the current tezos protocol (PsddFKi3) can legitimately declares "next block must be in a different protocol (Pt24m4xi)". At that point, if (hopefully when ;-) ) someone manages to produce a valid block in the new protocol Pt24m4xi, it propagates through the network and your node receives it. Your node will consider it as a valid candidate (as it is in the correct protocol) but, still, it has to validate it carefully. There are 2 options to do so:
1/ Your tezos-node has been compiled with protocol Pt24m4xi embedded "statically" so it knows how to validate the block and does it. This is the "you recompiled by yourself since protocol Pt24m4xi has been integrated into the code base" scenario.
2/ tezos-node has no clue about Pt24m4xi. In this case, it sends a message to the peer from which it received the block in the Pt24m4xi protocol saying "Hey you, you send me a block in the protocol Pt24m4xi, it means that (if you're an honest participant) you validated it. Therefore, you must know about Pt24m4xi! Please send me the source code of Pt24m4xi."
(If the peer doesn't, it is kicked.) If the peer does, the node checks that what it downloaded is the correct protocol by checking that the hash of the code source is the one announced, it compiles the received source code, dynamically links the produced library to itself and use it to validated the just received block. This is the "you didn't touch your tezos-node in a while" scenario. Your node will still keep following and validating the chain!
