I'm also facing the same situation.
The client is hosting their own email onsite (exchange server). We have finished developing the website and are ready to deploy. The hosting company (Company A) and the company managing their domain (Company B) are 2 different companies.
I have asked Company B not to modify the MX record. I will be soon asking them to forward web service to a unique IP that I purchased with DreamHost. I have modified the MX record for the domain (in the DreamHost CP) to forward to the current MX record onfile from Company B.
I will soon be asking them to switch the IP where web service is forwarded to the DreamHost unique IP.
Is there anything else I should take into consideration before making the switch? Am I even doing this the right way or am I crazy?
I'm a programmer/designer and I know the basics but I'm no expert first to admit that.