Switching to Fixed Fees as a practice - tales from the frontline

I've recently read Life without Timesheets by Hugh Williams which is a brilliant book and has

definitely opened my eyes about the way forward for our practice which currently uses timesheet

billing, yes I know how foolish that will be to most people.  The book gives excellent examples

of sample letters to use which we will be using also to move towards upfront fixed fees.  We are planning

to move within the next 2 months, to write to all our clients about our intention to move to fixed fees and

to then propose fixed fees for compliance work such as tax returns, accounts, payroll.

I know it will be a massive change in our practice but it has to be done, no other business would send you a bill

after they had done the work(well none that I can think of right now) so why should ours?  The case is so overwhelming

in favour of fixed upfront pricing that in my opinion it's a bit of a no brainer to change to this method.

I was just wondering if anyone had any experience of making this move, any recommendations/pitfalls

to bear in mind?  Any client feedback they had received for example what to do about existing debtors?

Thanks for any replies.