notebooks - Why can't I change the style of a cell repeatedly?

I ran into this when I was working with an example from the Help Pages of V9. Cells is a new function added in V9. The following appears to work the first time it is evaluated in a notebook with nb assigned to some appropriate value, say, EvaluationNotebook[].

Scan[(CurrentValue[#, StyleNames] = "Title") &, Cells[nb, CellStyle -> "Section"]]

However, if I try again to change the cells affected by the first evaluation to another style or back to the original style, nothing changes. Neither this

Scan[(CurrentValue[#, StyleNames] = "Text") &, Cells[nb, CellStyle -> "Title"]] 

nor this

Scan[(CurrentValue[#, StyleNames] = "Section") &, Cells[nb, CellStyle -> "Title"]]  

has any effect.

For other CurrenValue targets such as FontSize, it's easy to change the value repeatedly.

 Scan[(CurrentValue[#, FontSize] = 100) &,  Cells[nb, CellStyle -> "Section"]]
 Scan[(CurrentValue[#, FontSize] = 30) &,  Cells[nb, CellStyle -> "Section"]]

Answer

It appears that it only works once because it messes up the cell expressions it modifies the first time it is evaluated.

I started with a notebook with several kinds of cells but only one text cell, which had the cell expresion

Cell["Some text", "Text"]

I evaluated

nb = EvaluationNotebook[];
Scan[(CurrentValue[#, StyleNames] = "Section") &, Cells[nb, CellStyle -> "Text"]]

and everything looked good -- the text cell was reformatted to a section cell. However, the cell expression was now

Cell["Some text", "Text", "Section"]

It should have been

Cell["Some text", "Section"]

So I think we are looking at a bug.