Deca has said that they plan to have the Unity and Flash clients running side by side for a while. Also there will be access to the beta client through the supporter campaign. This means people will have quite a bit of time to learn to hack the unity client, and RotMG hacks probably won't have much or any downtime. Also, if Flash and Unity can coexist, it means that both the networking and fundamental game mechanics will remain unchanged, so all current proxy based hacks will continue to work indefinitely, and the Flash client could, with some effort, be updated even once Deca drops support for it.