Very little, if any, i would have thought because you are simply increasing the subnet mask rather than the other way round.
So any devices using the original subnet mask should still be able to talk to the router and the router will still think any devices using the original subnet mask are in the same network.
Edit - one thing you do need to be aware of though is the broadcast address. If devices using that subinterface are still using the original subnet mask then the L3 directed broadcast address they use will be seen by the router as a normal IP address because it's broadcast address has changed. So the router would drop the packet at the IP layer.
I completly agree with Jon, Infact we have done a migration of IP Schema of the sites where we have less impact but for around 150 sites more or less, we have first changed the ip address in the hosts which needs to be communicated and then changed the ip schema in the Network devices.
Engineers at some of the sites have given the wrong Mask in some of the hosts but still we haven't faced any pracket drop issue, where in the case is diffrent with the subinterfaces.
Still I suggest to take the down time approval a bit more than what we expect.
"Everybody is genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is a stupid."
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