incompatible initialization

I regularly upgrade my applications with portupgrade in FreeBSD (currently
4.7); postgresql 7.2 was upgraded to 7.3.1 and now does not start because
the data directory was initialized by version 7.2 and is incompatible with 7.3.
I guess this is one of the shortcomings of portupgrade, that this is not
done (or maybe cannot be done) automatically. Of course, the logical
expectation is that the upgrade is not done without specific directives.
So, now, my question is how do I update the database initialisation?
1) Do I erase the old ~/data directory and start from scratch or is there a
way to convert the initialisation?
Starting from scratch would not be a big issue as the existing databases
are not important - I hope) but I would like to, at least, go through the
exercise and see what I had there to begin with.

2) I do have to restore a 7.2 database from another machine to the 7.3
release on another machine - are there any insurmountable problems I can
expect?
Thanks much for any help.

PJ