Afrihost what exactly is the purpose of the secrecy around your upgrade plans, besides keeping your customers in the dark, thus further frustrating them?
A better approach would be to lay your cards on the table. Explain what your plans are, the progress you've made, and so forth. This would go a long way in helping the customers who would prefer sticking around (surprising, I know) to stick around for a little while longer.
I've even found a nice guide for you:
http://www.superoffice.com/blog/keep-your-customers-in-the-loop-and-keep-them-happy/
Here's a choice quote:
On the contrary, if you keep your customer well informed of all the trouble you’re going through to try and fix the problem, then you build loyalty, and create customer relationships that stick.