Migrations & Postgres: Change column not functional?

I’ve got a MySQL app, which I’m moving to a new server which is using
Postgres. This seems like the perfect application of migrations. (Note:
I
don’t mean “using migrations to switch from MySQL to Postgres” so much
as
“Hey, I’ve been using migrations all along, and this is where I cash
in”.)

There were some odd bugs when working with the site, and I traced it to
two
columns which had the “wrong” data types. Actually, they had earlier
data
types: Types that had changed in earlier migrations.

It appears that Postgres didn’t execute a number of “change_column”
instructions. It’s not clear to me, at this point, whether that was a
Postgres error, or something else. Has anyone else seen this behavior?
I’m
also wondering if this might be version-dependent: I’ve got Postgres
7.4.7and Rails
1.0…