Changing the structure of a csv file

Dear members,

I need your help to change the structure of csv file, A, to B in a new csv file, please.

I tried to record macro but it was not useful. There are some missing values in dataset (blank cells), but I couldn't fill them in format A because of blank cells in Feb and other non-31-day months.

I know how to run the code for the files in a folder, but for this part I need your assistance.
Reagrds,
Mori

[File A]
A1 B1 C1
A2 B2 C2
A3 B3 C3
A4
A5
>> change structure to >>
[File B]
A1
A2
A3
A4
A5
B1
B2
B3
C1
C2
C3

changing structure.png