Encouragement for most, and some questions for the knowledgeable.
I just had a 40 kWh battery from a junked 2019 Leaf installed in my early 2011 Leaf (vin ending 0577). The work was performed by a local independent shop, EV Rides. Installation was straight-forward requiring a plug-in gizmo to communicate to the vehicle computer that it has a proper battery (I'm intentionally obfuscating the details because this is not intended to be a how-to article). It is a functional success, with a couple monitoring systems qualifications. I have just a few hundred miles on the installation, but battery health, range, and charging appear normal. In mixed but mostly highway use I got 121 miles down to one bar remaining. Charging that to full on my 24A/220v home charger took 11 hours and consumed 36 kWh. I have an ODB sensor on order, so will be able to get more specific information soon. For now, I'm happy to report that there is hope for 1st gen Leaf owners who have what is an otherwise viable vehicle, but handicapped with a failing battery and manufacturer support. My conversion cost me $8k.
A couple minor issues and some questions for the cognoscenti: First, for reasons unclear and apparently unique to the early 2011 production run, plugging in the vehicle to charge does not activate the three charge lights on the dash. The battery is receiving the charge, but the indicators are not active. There is a goofy work-around if you want to activate the charge lights: Double tap the vehicle start switch, unplug the charger, then re-plug the charger. Yes, this is a minor issue and does not affect practical use (that I know of), but it would be nice to clean this up. I have charged the car both ways (with and without activating the charge lights) and there does not appear to be any functional difference to performance.
Second, the "(Est. Time) to 100%Charge" on the driver's display shows blank bars on the 240V and 120V lines. This is truly a minor distraction and is not a priority. I report it only as a data point and possible value for those who may understand and troubleshoot the system.
During my mixed-but-mostly-highway driving I lost my first bar at 44 miles, my second at 50 miles, my 7th bar at 74 miles, and my 8th bar at 83 miles. My last bar remaining brought me home at 121 miles. Battery temperature during driving appeared normal for the 77 degree outside air temperatures, if maybe 1 bar high. These numbers are almost meaningless, as consumption and performance is heavily factored by my variable right foot and the hilly region I live in. I include this purely for entertainment value.
So there you have it. A data point of encouragement for those with dying, early Nissan Leafs who don't want to junk the car for want of a modern battery and practical range. You can do your own research on the cooling, degradation, and performance of the 2019 40kWh battery pack and decide for yourself whether it is a fit for your priorities. It works for me.