On the 620 controller, with any firmware on a newer line than 6.1.x, you are hardware limited to 8 AP's whether they are RAP or CAP. So if you update to 6.4.x code, you'll be able to run less AP's than currently, not more. You're looking at upgrading to a new controller. The good news is that the new base model, the 7005, supports 16 APs. The 600 series controllers don't support all of the 6.3 and 6.4 features anyway.
As I understand it, the trade off with the newer code is that:
- Downside: RAP's and CAP's have the same value, so whatever controller you own, the old CAP hardware limit, is the new all AP limit. So admins can't skirt the AP limit by provisioning them as RAPs anymore.
- Upside: On 6.3 and newer there is License Pooling. So you don't have to buy all those duplicate licenses anymore for redundancy. Just set your Master controller as the "license server" for the master controller's domain, and your controllers can share licenses for redundancy now. So if one controller fails, it gives up the licenses it was consuming, and the backup LMS controller can take start using those licenses (just make sure that controller's hardware limit supports your total number of APs :-D ).