Are You Talking To Me?
My local library service is changing its online offering - and is doing so by having no online presence for three weeks while they switch it over. Worse still, their warnings about this were couched in terms of the catalogue being unavailable. That might mean something to the IT guys who wrote it but to this user, it certainly didn't suggest that all functionality would vanish and one would have to trek to the library to renew one's loans.
If you're talking to your users, then
1)Speak their language, not yours. Pre-warning is no use if it's vague. In fact, it's worse than useless - it increases the likelihood of irritation.
2)Remember that any project serves the user and not vice versa. Make sure the user experience is paramount.
3)Don't sugar-coat things - detailing the downside will not put people off as much as a nasty surprise further down the line.
In other words, make it easy for them to listen.