Gas boiler in living room cupboard

Very unusual to install a room sealed boiler in living room, but there are no current regs preventing this. As for the cupboard of your existing boiler, common practice to install a boiler in a redundant airing cupboard where once your hot water cylinder would have been. All modern boilers now are condensing. I would keep the existing location for your new boiler, it may require compartment ventilation, but your installer should be aware of this if required. In my first answer, I misread the information you provided, despite this, fitting a room sealed boiler in a cupboard under stairs is still acceptable practice, so no issues there, but still being classed as a compartment, new boiler may still require ventilation, as previously stated.