I would recommend against "simply" using a large net (read class A) and would
instead advise the use of as small of a block as possible. People
who move from a private network to the public Internet will face the
same problem as people moving from one network service provider to
another (if they renumber). If all future IP address assignments are
done along the lines of <CIDR prefix, host address part>, with host
address part assigned to meet local routing requirements, then people
who move over to the public Internet could possibly get by with only
changing their prefix. This will be an education problem in the short term.
It would be nice if tools like BSD ifconfig could take some syntax that
looked like <CIDR prefix, host address>.
Overall, it would make sense to draw these private prefixes out of a single
large block, such as 10.
cheers,
peter