Are you going to register new Catalyst 9120 AP's to the same 3504 WLC's or you bought a new WLC? If you are planning to register to the 3504 you need to upgrade the WLC to AireOS code 8.8 + or higher. TAC recommends to use 8.10.162.0. More info on Recommended AireOS Wireless LAN Controller Releases - Cisco
While WLC is the recommended way of management, you may explore the alternative solution EWC (new Catalyst WLC solution for AP's) as well. In this case WLC runs in the AP it self with limited functions. The Catalyst 9120AX and Catalyst 9130AX Series running the EWC support up to 100 Access Points and 2000 clients. Since 3504 is still supported by Cisco I would recommend to use the same WLC.
AP Migration is simple, you can replace AP to AP or based on your site survey locations you may install the AP's (latter is recommended). AIR-CAP2602I-E-K9 is not supported in 8.10.X AireOS codes, so plan accordingly to replace them during the migration window. But please make sure that you do not do salt and pepper deployments, group all catalyst AP's to one roaming domain and other Wave 1 AP's another roaming domain. (roaming domain is where the boundaries for client roaming without any coverage holes, typically a floor, building etc.)
