We had 100 year old Baltic in our last place. They had to sand them a lot to get them flat and smooth because they are very soft. The floor looked that flat afterwards it almost looked like lino!
Baltic is really soft so it doesn't take long for them to start getting roughed up and look a little more authentic ... they also seem to change colour overtime, so you may find that once the light has been on them a while they even out a little?
I also forund that after that sand they had taken them almost down to the tongues too, which wasn't the case with the harder boards in my current house.
I think you would be taking a fair risk to try and rectify it now. you would have to sand the coating off the newer boards and maybe stain them? But I think it would be fairly hard to intergrate and the coating would be uneven etc.
Sorry ... hope someone else has a better idea.