I have replaced a few floorboards that were too damaged for a new carpet to go over (hacked to death for central heating modifications over the years). The new boards a little thinner than the old ones and I have a 'step' of between 0 and 5mm in places. What is the best way to deal with this?
Find a better timber merchant, if it's a oddball thickness,some merchants will machine to your finish size.
I found Wickes did a 22mm pine floorboard which was a perfect fit for my bathroom, hard to find timber the right thickness.
You could place packer on the joists to raise the height to the same level and you may have to remove the tounge if it's tounged and grooved floorboards. Not ideal but if you have cut the boards and want to use them this is a work around.