After the 6.2-6.3 altertable is done - it'll give you a link to those 2 optional altertables.
One (replies) repropigates the "Re:Username" information that was removed in 6.2. The other (topics) will fill in the "last post by" information for all existing posts, the functionality that was added in 6.3.
If you skip those 2 - then old posts won't have the "Re: Username" information nor last post by information. New posts will propigate those fields as they go.
hope that helps.