Nothing has changed to my knowledge.
A formal IEP meeting requires at least 5 day notice (check state regs -- in some states it's 10), specific team members are required, yadda, yadda, yadda.
There's no such thing as an informal IEP meeting -- never has been since I've been involved in IEP meetings.
If your talking about an Amendment to the IEP without a full blown meeting, that's different. Minor changes can be made to an IEP as long as the parent agrees in writing.