Cisco Prime / 5508 WLC Captive Portals Setup

I've seen similar issues twice in the past.

The first time, it had to do with DNS resolution. I performed a DNS lookup on a client that was having issues, and realized that the client wasn't capable of resolving the URL that I was passing for the login page. This was because I was passing an external DNS server. Check that first. I resolved that by passing the IP in the URL, although you could create a cname that resolves to 1.1.1.1.

The second time, it was a certificate issue. Some default browsers won't show the splash screen if the user must accept a self-signed certificate. I would test that by manually browsing to the splash screen or login page from a client that does not automatically show it.

Hope one of those helps you out. I know that I was ready to pull my hair out when I saw this the first time.

I've never had to do a migration from 7.0 to 7.5 code in my environments.

The 2600 series APs were first supported in 7.2.110. http://www.cisco.com/en/US/docs/wireless/controller/release/notes/crn7_2_115_2.html#wp976967

The 5508 can run also 7.2. If you could get the mobility group/anchor to work with 7.2, then why you could downgrade it to 7.2 during the transition process.

You should be able to change CISCO-CAPWAP-CONTROLLER to your new 5508 controller so the 2600 APs will establish a connection with the new controller out of the box.
All CAPWAP and LWAP APs should remember the previous controller that they connected to and will attempt to establish a connection to those controllers before trying to go for the DNS lookup. See the CAPWAP controller discovery process

Thus, if one of your 1230 APs were to reboot - even if it does a DNS lookup for CISCO-CAPWAP-CONTROLLER (which would then return the 5508 IP), it should still connect to the old controller.

I would also assume that your new 5508 has SMARTnet? Open a TAC case with Cisco and see what they say!