user2306: The block size increase proposal currently under discussion would require all clients that process transactions to upgrade.
Otherwise, pre 0.11 clients would reject these larger blocks, leading to a possible double-spend attack against these merchants after Mar 1, 2016.
Will the Bitcoin developers use the alert system to warn these nodes to upgrade?

user25088: Yes, according to Gavin Andresen via Reddit.
Even if they didn't to do so, Bitcoin Core already has two other mechanisms which will produce automated alerts as the hard fork approaches: one which will take effect before the hard fork occurs, and one after (as per BIP-50).

2306: <p>The <a href="http://gavinandresen.ninja/time-to-roll-out-bigger-blocks" rel="nofollow">block size increase proposal</a> currently under discussion would require all clients that process transactions to upgrade.</p>

<p>Otherwise, pre 0.11 clients would reject these larger blocks, leading to a possible double-spend attack against these merchants after Mar 1, 2016.</p>

<p>Will the Bitcoin developers use the alert system to warn these nodes to upgrade?</p>
 25088: <p>Yes, according to <a href="http://www.reddit.com/r/Bitcoin/comments/2uuyzd/poll_should_the_maximum_block_size_be_increased/cocjhhe">Gavin Andresen via Reddit</a>.</p>

<p>Even if they didn't to do so, Bitcoin Core already has two other mechanisms which will produce automated alerts as the hard fork approaches: <a href="https://github.com/bitcoin/bitcoin/commit/dbca89b74b76610331d21656cd6747f5bf8375d6">one which will take effect</a> before the hard fork occurs, and <a href="https://github.com/bitcoin/bitcoin/commit/8fa9b5cc45c1826b00f305e91230c20df18885fa">one after</a> (as per <a href="https://github.com/bitcoin/bips/blob/master/bip-0050.mediawiki#safe-mode">BIP-50</a>).</p>
