Upfront Modeling is fine, documents describing the intended architecture are fine, and so forth. But the architecture, and our learning about it, can improve. Speculative software architecture should be made concrete and not of concrete.
Agile architecture comes from the behavior of agile architecting – hands-on master-programmer architects, a culture of excellence in code, an emphasis on pair-programming coaching for high-quality code/design, agile modeling, design workshops, test-driven development and refactoring, and other hands-on-the-code behaviors.
Source: Practices for Scaling Lean & Agile Development – Large, Multisite, and Offshore Product Development with Large-Scale Scrum, Craig Larman and Bas Vodde, Addison Wesley, ISBN 978-0-321-63640-9