Those phones will run industry-standard SIP rather than Cisco-proprietary native SCCP protocol, but there will be issues:
1. Getting the SIP firmware onto the phones. Normally they will download from the
Call manager TFTP directory if they are set up to do so.
2. They may not behave exactly the same on the Cisco Call manager, and may have
lost some functionality when moved to Avaya.
3. There may be licensing issues, both in DOING the conversion, and operating afterward.