change IP address of excahange and DC

[Oswaldo.]

I have a Win2k SP4 domain spanned over 50 sites. Im migrating my old
exchange server to a new hardware ( Mailbox server ) and adding an OWA front
end server, as well as adding a new DC and DNS Server. As my company uses
apps to send notifications and other stuff ( programming ) i was asked to
keep both IP address, the one of my exchange and DC ( the one that holds
Forest roles ).

Is this an option?
What issues should i take care of?
Should I do this or assign new address?

Regards
OSwaldo.