Replacing text in a file based on position

[cferdig]

What I am trying to accomplish is this in VB 2005:

Here is a sample of input data from a text file-

000000001111111122222233333333
000000001111111122222244444444
000000001111111122222255555555

What I would like to do is read 8 characters starting at position 23 and overwrite the first 8 characters in the line. The output would look like this:

333333331111111122222233333333
444444441111111122222244444444
555555551111111122222255555555

The characters are varaiable but the chunks I need to read and write are always in the same positions.
There are possibly several thousand lines and up to 10 seperate files in a folder that I need to process this way. I am new to VB 2005 and having a difficult time. Any help would be greatly appreciated.