[mySQL] Copy Colomn From 1 Database To Another

I'm trying to copy the contents of a column from 1 database to another. I need to do this as the contents in the current database have got messed up after an upgrade.

The DB structure

table forum_msgs
It contains a column called "topic_id".

Basically I need to copy over the topic ID fields from the "old" database into the "current" database into table forum_msg replacing any values that are currently in the column "topic_id"

I tried this but it doesn't seem to work correctly. Any ideas how it should be done?

insert current.forum_msgs
( topic_id
)
select topic_id
from old.forum_msgs