user58590: SegWit is meant to be backward-compatible. When a SigWit block is relayed to a non-SegWit node, the witness part is stripped. If this is the case, wouldn't it be invalid since the non-SegWit node sees no witness/signature in transaction input?
So output is changed to anyone-can-spend output? If so, what prevents it to be spent by anyone not the intended recipient?

user83104: 
If so, what prevents it to be spent by anyone not the intended recipient?

Most nodes (the ones who have updated) would reject this transaction. Every miner would reject this transaction out of fear that their block (and block reward) would be rejected.
You can fork off into a chain that spends those outputs though, if you wanted.

user208: 
So output is changed to anyone-can-spend output? If so, what prevents it to be spent by anyone not the intended recipient?

No, it is not changed.
A SegWit output is anyone-can-spend from the view of old nodes. This means that to them, it can be legally spent without signature.
To new nodes, it has a different meaning, and requires a (valid) witness to be spent. Nodes that care about SegWit outputs enforce this rule, and will not accept a blockchain that spends without such a witness.

58590: <p>SegWit is meant to be backward-compatible. When a SigWit block is relayed to a non-SegWit node, the witness part is stripped. If this is the case, wouldn't it be invalid since the non-SegWit node sees no witness/signature in transaction input?</p>

<p>So <a href="https://bitcoin.stackexchange.com/questions/53000/how-does-a-non-upgraded-node-read-a-segwit-transaction">output</a> is changed to anyone-can-spend output? If so, what prevents it to be spent by anyone not the intended recipient?</p>
 83104: <blockquote>
  <p>If so, what prevents it to be spent by anyone not the intended recipient?</p>
</blockquote>

<p>Most nodes (the ones who have updated) would reject this transaction. Every miner would reject this transaction out of fear that their block (and block reward) would be rejected.</p>

<p>You can fork off into a chain that spends those outputs though, if you wanted.</p>
 208: <blockquote>
  <p>So output is changed to anyone-can-spend output? If so, what prevents it to be spent by anyone not the intended recipient?</p>
</blockquote>

<p>No, it is not changed.</p>

<p>A SegWit output is anyone-can-spend <strong>from the view of old nodes</strong>. This means that to them, it can be legally spent without signature.</p>

<p>To <strong>new nodes</strong>, it has a different meaning, and requires a (valid) witness to be spent. Nodes that care about SegWit outputs enforce this rule, and will not accept a blockchain that spends without such a witness.</p>
