Calling all GL Experts

OK....Do not ask my why, but we have a request from our French sub to
change their chart of accounts.  They are currently setup with 1 level and
1 extension code level.  They would like to change this to be 2 chart of
account levels and no extension code levels.  My questions are therefore:

1. What is the best way to do this?
2. What effect will this haveon existing data?
3. When should we do this?  Would it be something that should only be done
at year end or can it be done in the middle of the year?
4. What happens if I switch off extension codes anyway?  Will this mess
anything up (even if I do not add the extra level)?
5. Anyone done it and what she we watch out for and what would the steps
be?

Thanks for all the help.

Cheers,
John