Sounds challenging on a number of levels. :)
I have been the Chair of a similar local activity group that could also be seen as having some members who might have an arguably similar reluctance to change. That group has less than a tenth of the members under discussion in this thread, but I had a similar experience when I first took on the role. Our committee had to spend a good proportion of time on applying some data protection principles to the annual data that had been collected the exact same way for many years (and a complete copy of which was being sent back to all group members almost in its entirety).
In the end, as well as stopping the mass mailouts of personal data, we migrated the group away from using the postal addresses that had historically been collected as a primary identifier but were no longer really necessary, and switched over to email addresses, which are far more practical for the modern group activities.
I found that the majority of reluctance to provide email addresses faded away when I spent some time explaining that, as Chair, I felt I had a responsibility not only to the current membership, but also to the volunteers on the committee and - crucially - the future membership and future committees. I found that when you describe the extra voluntary effort that is required to maintain such historical data / processes (e.g. dealing with members by post / telephone, rather then email), and then ask complainants either directly or indirectly, whether they feel strongly enough about the decision to volunteer to be membership secretary and take on that role, then those members can be brought round. I've also found that whilst some members can be extremely vocal, often the vast silent majority think very differently, even if you haven't heard from them.
If I was on such a committee that considered the move to a new primary identifier was the best move for both the group and the volunteers on the committee, but it seemed like there may be a chance of disapproval from some, I would likely put the decision to the members and ask them to vote on it at the AGM, after explaining the pros and cons of what that decision would entail. If you have a vote of approval from the membership, I think that gives you a mandate to proceed with what is best for that group.
My experience of committees is that to get anything done, it often requires as much effort put into convincing and explaining to other conservative members, as much as it is time spent implementing. It's often the case that volunteer committee members don't really want to have to deal with this additional pain, and that can ensure that groups remain stuck with out-of-date processes, until something happens which forces a change.
Good luck with it all, anyway. I hope your members appreciate your diligence!
Peter.
P.S. In the end, we only had one couple who remained anti-email (and internet in general). Their experience didn't really change much though - we continue to accept printed out membership forms, so we