LONDON (Reuters) - British property funds are set to remain frozen for months as the market is impossible to value due to the coronavirus crisis, and some may need to change structure to survive, industry sources say.Ten big open-ended property funds tracked by Morningstar, with a total of 6.5 billion pounds ($8 billion) under management, stopped investors from getting their money out in mid March, saying valuers could not accurately assess real estate assets in a plunging economy.With question..