Just a note concerning Nissan/Infiniti leather.
There's two different grades of leather used in the early aught models. There's the premium "supple" type and the "sport" leather option. They're very different in texture.
The "sport" leather is a cheaper grade though they look identical. It's firmer and what I suspect you had as OEM. It sounds like the replacement may have been the more premium style. It's hard to differentiate unless you have them side by side, which you have.
BMW has the same issue and I'm sure other makers do too. When I worked at the Bimmer dealership we had to use the VIN to determine what type of leather options were available, and then the archived Monroney sticker to further narrow it down. There were approximately 30 different options encompassing four styles/grades of leather.
So if as I suspect they're two different grades of leather, you're not going to make one more like the other. But if I'm wrong and it's just that one is softer than the other, there's only one product I've ever tried in 40+ years of automotive detailing that does what it states:
Leather Master's VITAL. If you are in need of intense softening of the leather this is the only way to go.
My recommendation would be to purchase the Leather Master Bundle off Amazon for $35: Leather Bundle
1.Clean all of the seats twice with the Soft Cleaner.
2.Then use a good bit of the VITAL on the seats that need to be softened. My recommendation is to do one round of application and let that soak in overnight/24 hours. Then buff it out and do another round and let it set overnight once more.
3. Buff out the second coating, then apply the LPC (leather protection cream) to all of the seats. Let it sit about 15-30 minutes and then buff it out. Wait overnight before you drive it.
If you try to drive it a couple of hours after the Creams have been buffed out then you're going to get an issue where the sweat and the oils from your body are intermingling with the still-moist layer of LPC which is going to hinder its ability to cure properly.
I've tried almost everything on the market, and that's not just hyperbole.