one major version difference is officially supported (feature set will be lowest version), two major version difference is not. It might work, don't bother reporting troubles (even none SCN related). Trouble will be discarded instantly, you will be told to fix the version difference issue. I suspect that Avaya will actually treat the 8.1 release as a major as well, making it a three release difference.
From the knowledgebase:
Software Level Interoperation
Small Community Networks is supported between systems with the same major software level or one level of difference in major software level. For example between 9.1 and 9.0 (same major level) and between 8.0 and 9.0 (one major level of difference).
This option is intended mainly to allow the phased upgrading of sites within a Small Community Network. It is still recommended that all systems within a network are upgraded to the same level where possible. Within a Small Community Network including differing levels of software, the network features and capacity will be based on the lowest level of software within the network.
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