I'm comparatively new in FX market, but as having a trading experience in various markets over 20 years, new and interesting concept for me )
I have in the past had a spike in price that gave me a nice profit.
The broker then deleted the spike and removed my profiting trade.
That spike was removed from their chart and I closed my account with them.
So yes, brokers can change their past quotes.
Welcome to Forex Market. :-)
Yes they returns as recent Roboforex return all profits and losses as they had problem with LP and later they fixed price again.
Some broker also returns looses when they offer negative balance protection like FXCM did on 15 Jan 2015 but they took bailout package hope again they will not offer this to clients.