ARIN allocating /20 netblocks?

> Ok, so I only lose 16 T1 customers instead of 1024.

If you lose 16 T1 customers during a renumbering of a /20, either you
don't know how to renumber, or they were already unhappy for some
other reason.

If you tell your customers "You need to renumber because evil forces
are colluding against us," they won't be very amenable... try
explaining that they'll get better connectivity.

The social and technical problems of renumbering a /20 are nonzero but
not large.

I've done it.
Shields, CrossLink.

Not always. I know of at least one case where a customer cannot (will not)
renumber out of a /24 because they (without commenting on the intellectual
prowess of this particular act) shipped a million CD's with the .128 IP
address in that class C hard coded into their game. Sure, it's only one IP,
one machine to renumber, but...