All of the online trust vendors that I'm aware of provide living revocable trusts, which means that the settlor is able to amend the trust. How one makes the changes depends on the type of change being made and the materials provided in the trust deliverables.
I'm not sure what 199Trust includes in their materials, but NFA PRO
http://nfapro.com/ provides forms for adding and removing trustees. Changing beneficiaries or a successor trustee requires an amendment to the underlying agreement, and can have unintended consequences. For that reason it's something you might want to consider having an attorney assist you with.
One of the other benefits of an NFA PRO trust is that you can name your "descendants" as beneficiaries, thus eliminating the need to amend your trust each time a new child or grandchild is born. You may also name a spouse as a co-settlor and trustee, or just a co-settlor.