Can I switch between the Ultra and the Half Marathon after entering?

No. Two Oceans does not allow upgrades or downgrades between the Ultra and the Half under any circumstances, and your entry can only be used for the distance you entered.

The only way to change events is through the official substitution process: substitute out of your current race, then substitute into the other race if a substitution entry is available. Substitutions are like-for-like by distance and residence category, and Ultra substitutes need a qualifier (and, for South Africans, club membership).