Wait.
This isn’t that simple, changing it can break things.
If you are on 2.0, just change the size of allowed post characters to something bigger than 65536 and it’ll convert, provided that you aren’t using full text search, because full text indexes with mediumtext aren’t supported.
Don’t bother with largetext, that just eats space and you can’t post a post big enough to fill even a mediumtext without reconfiguring MySQL anyway.
If you are on 1.1, upgrade to 2.0.