pick up upgrade?

Well, there certainly are more options. Let me be the first to ask the obvious, do you like the bass as it currently sounds? If you don't, upgrade away. If you do, I guess this will be an interesting but pointless conversation

 

I'd be very reluctant to give up 80's DiMarzio's. They ruled the world back then, top of their game!

 

This. Ride those old Dimarzios ‘til they die.

 

I've had vintage PAF Model Js and brand new ones and the sound is very much the same. Aside from aging components, you wouldn't notice much difference in the same exact model.

That said, the Area J and Ultra J sound a lot different than the Model J.
Area is more like the traditional Fender sound, but with more output and noiseless.
The Ultra is very scoopy mid, great for that Marcus Miller sound right out of the box.

Personally, I like the Model J and if I were in your shoes, I'd stick with how it is. That sounds like an awesome setup to have.

 

To the first question: If you are happy with the tone of your current pickups, then "no".

To the second question: It depends. Quality control has improved. There have been innovations and design improvements. However, good tone is good tone. That never really changes. If your pickups have made it over 40 years, the only reason to consider different pickups is if you're looking for a different tone.

 

I lack personal experience, but I’ve heard old DiMarzios either sound the same as new, sound darker, or sound brighter

I’d bet they sound about the same as always.

There are many options these days, but given that Model Js remain quite popular, I think it’s fair to say they sound different, but not necessarily better than Model Js. What seems to have changed, though, is that some newer split coil Js sound more like single coil Js (just without hum), if that’s what you’re after...