You can perform a redirect to the new address using IIS.
Windows Server 2008 For a
Configuration in Which SSL Is Not
Required on the Default Web Site or on
the OWA Virtual Directory in IIS 7.0
To use IIS Manager to simplify the
Outlook Web Access URL Start the
Internet Information Services (IIS)
Manager snap-in.
Expand the local computer, expand
Sites, and then click Default Web
Site.
At the bottom of the Default Web Site
Home pane, click Features View if this
option is not already selected.
In the IIS section, double-click HTTP
Redirect.
Click to select the Redirect requests
to this destination check box, and
then type /owa.
Under Redirect Behavior, click to
select the Only redirect requests to
content in this directory (not
subdirectories) check box.
In the Status code list, click Found
(302).
In the Actions pane, click Apply.
For the new settings to take effect,
start a command prompt, and then run
the IISReset command.
Source
You can also modify the properties of the virtual directory, but I'm not sure of all the consequences of that.
To use the Exchange Management Console
to modify the properties on an Outlook
Web Access virtual directory In the
Exchange Management Console, locate
the virtual directory that you want to
modify by using the information in
step 2 or step 3.
If you are running the Mailbox server
role on the computer that is running
the Client Access server role, do one
of the following:
To modify an Exchange 2007 virtual
directory, select Server
Configuration, select Client Access,
and then click the Outlook Web Access
tab. The default Exchange 2007 virtual
directory is /owa. To modify a legacy
virtual directory, select Server
Configuration, select Mailbox, and
then click the WebDAV tab. The default
legacy virtual directories are as
follows: /Public, /Exchweb, /Exchange,
and /Exadmin. If you are not running
the Mailbox server role on the
computer that is running the Client
Access server role, select Server
Configuration, select Client Access,
and then click the Outlook Web Access
tab.
In the work pane, select the virtual
directory that you want to modify, and
then click Properties.
In the virtual directory properties
dialog box, select the tab that
contains the properties that you want
to modify.
Make the changes that you want.
Click OK to save your changes and
close the properties window.
Source
Obviously, read up and apply what fits for your given situation.