Furnished Holiday letting Ceases and Rental Income Starts

I have a client who has purchased a caravan and is entitled to claim a large annual Investment Allowance on her rental income. This will generate a significant tax rebate this year. However what will happen from 2010/2011 when the Furnished Holiday letting rules cease to apply. Will this be treated as a cesation of trade resulting in a large balancing charge based on the Market value of the caravan. If so I believe I should warn my client that she faces a tax bill in the future because the rules have changed. OR when the rental rules change can we simply transfer asset at Written Down Value ie NIL Would any future sale then be treated as a capital gain with a purchase price of ZERO.