Here's some questions: WHose confession? Do we get to go shopping in the mall and pick up the one we like in our 20s, and switch to another one in our 30s?
If we style ourselves doctors of the church, can everyone in this thread devise their own confession by mutual consent?
No you can't pick your favorite confession, except indirectly by deciding to be part of a particular church tradition. Let me give you a view of what it means to be confessional that is different from the other participants in this group, but which represents many (maybe even a majority) of Reformed Christians, at least in the English-speaking world.
For us, being confessional is a consequence of being a church. God doesn't leave us to figure out things on our own. He grafts us into his body. He gives that body the authority to make decisions. That's what the power of the keys is about. This was a rabbinical concept: the authority of a rabbi to make authoritative interpretation of Torah. See Mat 16:18-20.
For that reason in the Reformed tradition, theology is always done in community. That community includes great theologians of the past. We don't always accept Augustine's views, but he is part of the conversation. So is Calvin, Barth, and current writers such as Wright.
Confessions are a way that community expresses its faith. In the 16th Cent it was used apologetically, as a way to say "our enemies are making all kind of false claims about how we are rejecting Christianity, so here's what we really believe." Confessions are used for teaching our own members. And they are used to establish the distinctive position of our community. Members, and particularly leaders, are expected to be part of that community and to be guided by its confessions.
This does not, however, mean that the community will never change, nor that members are required to agree with every sentence of the confessions. The Presbyterian tradition, from the early colonial days, has allowed disagreements with the confessions, requiring only agreement to the essentials of the Reformed faith.
The Presbyterian community didn't issue any new confessions between the 17th Cent and the 20th, although the American church did change Westminster to fit the circumstances in the US. In the early 20th Cent it issued some "clarifications" of the Westminster Confession which in my view changed some of its provisions. E.g. it explicitly rejected limited atonement. Since then, there have been several new confessions, Barmen, the Confession of 1967, and the Brief Statement of the Reformed Faith. There's a proposal (which I support) to add Calvin's French Confession. (Incidentally, I agree with the PCC document below that it's a bad idea to change historic confessions. Despite the weirdness of adding clarifications that change the content, I think it's better to do that than the tamper with the text of the confession.)
The Presbyterian Church in Canada has a nice treatment of what it means to be confessional. It looks at history, the views of other churches and the two approaches currently common within the Reformed tradition:
http://presbyterian.ca/?wpdmdl=278. I commend this document to your attention.