Talk to the client, explain that the free server upgrade was obviously dependent on them using you for the SEO work. Tell them they are free to use someone else for SEO, and if the results do not live up to the promises, they are welcome to come back to you, but you won't be dropping your prices.
In the meantime, the free server offer is no more, so tell them to cough up the extra or stay on the slow server.
If your client is determined to get a cheaper deal elsewhere at the risk of losing a valued partner with proven results, let them go and learn the hard way.
It's hard not to take stuff like this personally though, especially if you are a one-man shop and feel that you've developed a real partnership with the client.