wouldn't LCD be downgrade? i never owned plasma tv, but AFAIK biggest advantage are perfect blacks, so I guess OLED should be more natural upgrade path?

OLEDs have very good blacks, but as soon as a pixel needs to emit any light at all, it will be significantly brighter than what a regular IPS pixel can emit at it's lowest brightness.

LCD with full-array local dimming can get really close. It also gets much brighter than OLED. It's a much tougher decision than it used to be.

Also depends on the TV you upgrade to: replacing a 50" plasma with an 84" LCD will do nothing to decrease your energy usage.

While plasma uses a bit more energy than LCD it's not that big of a deal (especially if you've got one of the last gen plasma panels) if you don't use it as some sort of moving wallpaper but just turn it off when not in use.

Probably still would, if you interpolate this graph: https://www.rtings.com/images/power-consumption.png

Still, wouldn't recommend buying a huge TV. Jevons paradox and all.

Depends on the generation. I've got one of the last plasma's made (2013 model), it's a 42 inch panel and has a max consumption of 180W (only when at full brightness viewing a white screen).