There are lots of good plug-and-play LED replacements for H11 halogens these days. Just don't go China-cheap. Here's a very professional and fair appraisal of bulbs from the major players:

My personal recommendation would be Diode Dynamics. They came in the top 5, are a sponsor of the forum, and are the only top brand of LED made in USA.

 

No retrofit is necessary. Installing an LED into the existing assembly should be a slam dunk. The only issue is making sure the LEDs are "clocked" (oriented) properly and are still aimed OK.

 

I was curious bc my tail lights are LED, but why didn't Nissan do that with the headlights? It is what it is...

 

LED taillights suck a whole lot less juice than headlights, and consequently produce a lot less waste heat. The OE LED headlights have a heatsink inside that's the size of your fist, plus a very complex driver/booster board. They're also ridiculously expensive because Nissan only makes a fraction as many of them.