I have managed to recreate the issue in a test environment. With this kind of change the client assumes that the rest of the network is on a newer version and perhaps an upgrade is required. I need to change this so that it treats this as a hard fork and marks the old blockchain as misbehaving and drops them from its list of connected peers. Then we can do this again.
Has this issue been addressed? The Hiro market continues to freeze on Poloniex, and when I asked them why this continues to happen, they are saying the issue is something that needs to be fixed by Hirocoin. Mintpal seems to be trading hiro still (well, dumping, but I digress) so not sure what's going on here.