Question, if you upgrade the RFS4000 (currently running 5.8.0), why would you only upgrade to 5.8.4? What model APs
To answer your question though:
If you upgrade the RFS4000 but do NOT upgrade the APs, the APs will still be adopted, but you will not be able to manage the APs (you will not be able to use the RFS4000 to push out configuration changes to the APs)
If a site loses power, the APs will function normally. They will continue to operate using the version of WiNG they are running (5.8.0).
Regarding the upgrade process taking 5-7 days:
If it will take this long, my recommendation would be to use the RFS4000 to send the firmware updates to all the APs first. Instruct the RFS4000 to *NOT* reboot the APs though (this is a configuration option when upgrading APs). Wait until the firmware has been received by all of the APs. During that time, there is no disruption of service.
Once that process finishes, you can then manually instruct the APs to reboot, which will then complete the AP upgrade process.
There are multiple options when upgrading though. This is just one way of doing it. It just depends on your requirements.