Jeremy,
There are two ways to do this, one way is to tar up the account &/home/XXXX& in questions outside of the accounts directory, delete the account in WHM , recreate the account as an IP based in WHM then untar the data back in the new IP based site.
A second solution that I had found in a post a while back is below, I have not tested it but it looks like it is correct.
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Change non-IP to an IP Account
1. edit httpd.conf and delete the NameVirtualHost line for that domain.
2. change the IP from the shared to the dedicated.
3. add the IP to /etc/ips (it may already be there)
4. go to WHM and add in that IP address. (it may already be there)
5. Create an entry in proftpd.conf with that IP address and domain name.
6. Edit DNS for that domain to reflect the new IP address.
7. Restart proftpd and apache.
You'll probably want to confirm their zone files reflect the correct
information, as well.