Equating HPS lamps to LED lamps can be tricky. There's a lot of things to consider to get it right.
Here's what's happening in the existing HPS system. Although the bare lamp emitted around 25,000 lumens, your fixture is only about 70% efficient, so you never got the full effect of the bare lamp. HPS exhibits rapid lumen deprectiation, so that after about 12,000 hour of "on" time, it's only emitting around 50% of the original lumens. HPS delivers that tell-tale yellow tone, which is between 1800K and 2200K.
To find an LED lamp that's relatively equivalent, you can start by multiplying 25,000 x .70 =17,500. Then you can guess-timate the lumen depreciation by choosing a value between 100% (full light when the HPS was brand new) and 50% (realistic level for an older HPS lamp). If you split the difference at 75%, you get 13,000 lumens.
If you choose to use a corn lamp, you need to double the lumens, so you'd need a lamp that gives about 26,000 lumens (look at that! We're back to where we started from!) because the lamp itself will reduce the fixture efficiency down to approximately 40%. This is because the corn lamp itself will block all of the light that goes backwards into the reflector from bouncing out through the lens. This is the main reason that corn lamps are the scourge of the lighting world.
Finding a corn lamp that delivers that many lumens isn't hard, but finding one that's that powerful that will fit into your fixture will be really hard.
If you like the yellow tone of the HPS lamp, you can find LED lamps that reproduce that color. They're pretty common. If you prefer the cooler and more "white" tones, then what you'll notice is that you need fewer lumens to achieve a similar affect. This isn't because the cooler tones are "brighter" than the yellow tones (that's a common misconception). The cooler tones just make it easier for your eyes and brain to work properly.
What you should look for is a "filament" lamp, with the same screw base that you currently have, in a CCT between 3000K and 5000K (your choice) and somewhere around 10,000 to 13,000 lumens. Filament lamps are clear, so they utilize the reflector in your fixture and run very cool, so heat isn't normally an issue.
All that said, new LED floodlights are so cheap these days, that it would make just as much sense to ditch the old battleship and get a new slim, sleek floodlight.
Good luck,
CD
