If you are running a 2500 series controller on version 2.0.0.0, you are likely dealing with one of two scenarios:
He needed a specific tool: the Field Upgrade Software (FUS). This wasn't just an OS update; it was a low-level recovery image designed to rewrite the bootloader and partition tables. Most engineers ignored it until everything went wrong. air-ct2500-k9-2-0-0-0-fus.aes
Ensure you are on a compatible base version before jumping to the FUS. Draft 2: Internal Work Memo/Slack Subject: Planned Firmware Maintenance: Cisco 2504 WLC The Cisco AIR-CT2500-K9: A Deep Dive into Version 2
If you need this file for recovery, check Cisco's Software Download portal under "Wireless > Wireless Controllers > 2500 Series" but note that access requires a valid service contract. For lab use, consider virtual controllers (vWLC) instead of fighting with legacy FUS files. Sequence: Ensure you are on a compatible base