CHanging IP's

Hello,

I have 6 IP addresses.
xxx.xxx.xxx.151 = NS1 ( = main shared ip)
xxx.xxx.xxx.152 = NS2

xxx.xxx.xxx.153 --> 156
I use the last 4 ip's (153 --> 156) for customers who want to upload/view there side before there newly registered domainname is available.
(I use the NS1 & NS2 ip's when I register the domainnames)

Once there domainname is activated I want to change there IP address into the main shared IP of the server (151) but when I do this and they visit there website, they got the standard server message in there browser and not there side anymore.

I do this via: Account functions --> Change site's ip address

What am I doing wrong ??