Different sysinstall labelling behaviour when run post-install vs. during install?

[Darren Pilgrim]

Yesterday I was upgrading the disk in one of my workstations.  Since I was
planning on growing the filesystems for my FreeBSD install at the same time,
I needed to manually create a new disklabel.  I used sysinstall for this,
but ran into a problem: sysinstall doesn't allocate the partition letters,
with 'a' first, if you aren't doing an initial install.  I ended up using
the "auto defaults" option to create the 'a' partition, then remove
everything and create them as I wanted.  I'm glad to say that it worked out
just fine, but this problem raises a couple of questions:

Why does sysinstall behave differently when run post-install?

How do I manually specify or change the partition letter to use?