FC3 to FC5 upgrade - fails on transaction disk space

[B Wooster]

I've a FC3 system, and am trying to go through the "upgrade" option on
the FC5 installer.

I've 2.3G free on root (/), and 9.3G used, which I thought would be
enough, but 10 minutes into the upgrade, after copying image to disk,
and then something about transaction log, it errors out.

Any way to fix this? Can I make the upgrade use some other partition
for temp space? I've /tmp in its own partition, has 3.5G free.

I think the error message went like this:
There was an error running your transaction, for the following
reason(s): insufficient disk space.