Implementing backwards compatibility isn't a snap, but it should be possible. However that idea about alerting ppl using pre 1.25 versions is catchy. But how?
An idea would be to have a 1.25 server send a 1.20 (!!) level containing a text such as:

"Your version is outdated! Please get an update as soon as possible. You can get it from the following locations:"

Adding backwards compatibility in theory is kinda easy. Just turn off all the NEW options upon connecting to an older server. But what's the fun in that?