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Redirection is when the initial GW tells the sending host there's another GW with a better path to the destination.
Jon raises a great point that my initial mention of using redirection might not work because there's two different subnets involved. I've already noted I wasn't sure what the GW would do, i.e. whether it would send a redirection message if a 10.1.11.0/24 host sends to the lower half of 10.1.10.0/23. I've been assuming the GW is changed to 10.1.11.1/23, so it will see the destination on the local subnet. This is why I'm unsure whether it will send a redirection message.
However, if GW did send the redirection message, then Jon's mention about the sending host creates another potential issue, as the sending host (still on 10.1.11.0/24) would still see the redirection IP as on a different subnet. So, I'm also unsure how the sending host will respond.
It's something you could try, but even assuming it worked, unless we dug through the RFCs and could show it ought to work, it's not an approach you would really want to rely on. Even if the RFCs said it should work, it's unusual enough that it's the kind of thing that you're likely to find a bug in its implementation.
That said, what you could do though is change your GW to 10.1.11.1/23, not allocate (initially) any IPs not also in 10.1.11.0/24, and convert your devices to the /23 mask (if any are DHCP, that would be easy for them). Once all your devices are using the /23 mask, you can start to allocate IPs from the bottom half.
If you need additional IP space NOW, you could either permanently allocate them from a new address block, and not plan on changing the hosts in 10.1.11.0/24, or temporarily allocate them in a different address block, not using the lower half of 10.1.10.0/23, while you migrate as described in the prior paragraph. Once the migration is finished you migrate those new hosts into 10.1.10.0/23 and remove the temporary network.
The only advantage of the last, it gets all your hosts, eventually, into 10.1.10.0/23 (which may, or may not, be a good thing).
