Whilst I liked working on the Meridian/Norstar/BCM, Nortel really did keep the core of Norstar running too long, they needed to move away from that, drawn a line under it and should of started from the ground up with the BCM when it moved to the Linux OS, the phones were really reliable, but unfortunately the idea of using a HDD plagued it with problems, in fact this HDD failure represents 80% of faults I attend.
Such a pity though, they played a massive part in telecoms history.
A little bit on the original question, all Rls6.0 systems can be expanded, have software support and updates until 1st October 2015, extended software support will be then given for a further 3 years until 1st October 2018, see the following
IP Office Rls7.0 brought support for the Nortel handsets M7xxx/M7xxxN/T-Series using the TCM8 base card, DS16A and DS30A expansion modules, the expansion modules even have the RJ21 connectors to aid an easier transition, they just missed a massive market by not providing any license migration what so ever, this makes it quite cost prohibitive to migrate a license heavy BCM. Having said that, the handsets work really well on the IPO, they retain a lot of the features of the BCM, and add most of the IPO features, makes for a nice blend of the two. Also 11xx and 12xx handsets are also supported, but they convert to SIP based handsets on the IPO, but in my opinion the user experience is not great, no doubt as SIP gets better into the future, this might change.