Again about spread

 
From history- Slawa said:
"spread is constant and cannot be changed during backtesting".
What about this , when when broker quotes 10 pips spread and suddenly spread is changed to 5 pips. How can i accurately backtest for now, without changing spread from symbol properties. Is it soo hard to to make it changeable?
 
we cannot model temporary changing of spread by the broker
Reason: