Can I change the public IP of a website on dual wan router without losing customers?

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Hi,

Not sure if this is the right forum, but here is my question.

We host a secure web application in house with address:
https://myapp.mydomain.com
Our main website is hosted by 1and1 web hosting.
http://www.mydomain.com
I setup 1and1's name servers to point the subdomain 'myapp', to the
public static IP of our T1 at the office. The web traffic is is then
forwarded by our Linksys RV082 router to our internal IIS 6 box hosting
the web application. Everything works great.

We are switching from the T1 to an SDSL solution. The RV082 router has
dual wan interface, so I can attach the SDSL to the 2nd wan port
without disrupting the T1. I then have two internet connections and two
public IP addresses. The 80 and 443 traffic is forwarded over either
wan interface to the the internal IIS 6 box hosting the web app.

I want to make the name server switch to point
https://myapp.mydomain.com from the T1 IP to the SDSL IP without
disrupting any users of the web application. I realize that there is a
12 - 48 hour dns propagation delay when changing the IP's of a public
dns record. However, given my setup, I believe that every user will
correctly get to my web application during the propagation period
because they will be sent to either the T1 or the SDSL public IP. A
given user session with the our web application will last about 30
minutes.

Can anyone provide insight as to whether this scenario will work, or if
there are any potential problems?

Thanks,

Ed J.

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