upgrade/v2406-User-Upgrade-Guide.md

@@ -9,6 +9,37 @@
 
 [[_TOC_]]
 
+## Breaking changes
+
+### Finite Area
+
+The finite-area infrastructure has been revised to have its own
+registry, with provisioning for multiple area regions. As a
+consequence of this change, the finite-area fields and controls are
+now located in a different set of directories:
+```
+<time>/finite-area/area-fields
+constant/finite-area/faMesh/
+system/finite-area/faSchemes
+system/finite-area/faSolution
+```
+
+This is unfortunately, a **breaking change** since conditionally
+handling a variety of different possible file locations internally
+within the code rapidly cascades out of control.
+
+The `bin/foamUpgradeFiniteArea` script is provided instead to assist
+the user in upgrading a case setup.
+The script will examine the following directories:
+
+| directory     | actions |
+|---------------|---------------|
+| `0/`, `0.orig/` | looks for `areaScalarField`, `edgeVectorField`, ... fields |
+| `constant/` | looks for a `faMesh/` directory |
+| `system/` | looks for `faSchemes`, `faSolution`, and `faMeshDefinition` files |
+
+See `foamUpgradeFiniteArea -help` for usage.
+
 ## Deprecation and Removal
 
 ### Deprecated Models