Hi Tracy!

Making some assumptions:
- You're in detailed mode
- The project is more than one floor
- Your floor-to-floor height is not necessarily that 8-10ft height (?)

Here's what I'd do:
1. Acquire details/drawings to reference so you don't waste time later
correcting your present efforts.
2. Right-click in the component tree and copy the existing construction
assigned to the walls under discussion
3. Rename the (identical) old and new constructions something easy to
reference like "8in Wall"/"6in Wall"
4. Double click both constructions and modify the layers to match the
two actual constructions as necessary. This may be as simple as
adjusting the thickness of a single material for one construction.
Again, work from reference-able details if at all possible.
5. Observe which construction is currently assigned to all walls, then
re-assign walls to the other construction as necessary to match reality
for walls above/below the elevation where wall "steps." Re-assigning
constructions is fastest in spreadsheet view, but the 3D view is helpful
to identify which groups of walls need the re-assignments more easily.
What I'm getting at is you'll spend the least time if you force yourself
to make use of both views when making such edits, especially in a large
model.

If there is not already a clean break in your walls for that arbitrary
height at which the thickness changes (i.e. matching the floor-to-floor
or ceiling/plenum elevations), you may instead wish to define a 3rd
construction that represents a weighted average of the two thicknesses
over the stepped area. That approximation would be the fastest fix.

The alternative would be to more substantially modify the height of the
existing walls and create new ones at the critical thickness-changing
elevation - more to explain than I have time for right now, but I can't
immediately think of a scenario where that would really be
time-efficient...

Hope this helps!

~Nick

NICK CATON, P.E.