Thank you for the replies.
I appreciate the fact that the current will double, but the duty cycle is very small so heating will be insignificant.
I am also concerned that there may be too much current for whatever switching device is fitted to the power supply.
There is no active PFC circuit. The power supply is quoted as working from DC as is.
The load is not an LED. I need 3.8V and the load can vary from nothing to nearly 2A. The HV9910 wouldn't really be suitable.
It could be easier for me to use a buck regulator with an external MOSFET. I was concerned that the duty cycle would be very small, and at low load currents the MOSFET would only be on for an extreemly short time, and therefore difficult to control.