Converting non-null unique idx to pkey

I'm preparing a fairly large 7.4.6 DB for trigger-based
replication. I'm looking for ways to minimize my impact on the
existing schema & data and uptime. This replication solution
requires every table to have a primary key. Rather than adding
a new key column and index for the pkey, it's appealing to just
to reuse existing unique indices on non-null columns. Are there
are any known or obvious gotchas associated with transforming a
unique index on a non null column into a primary key via this
sql?

update pg_index
set indisprimary = 't'
where indexrelid = <my non-null unique index oid>

TIA.
Ed