I have a '95 USDM Celica ST that's been converted to HIDs. I like the look for the main lights but I don't like the high beams taking a few seconds to warm up. What would I need to look at for upgrading the high beams to LED?

There really isn’t anything to it. Unplug the HID’s and ballast and plug in the LED and you’re finished. No different than simply replacing any other light bulb.

Due to how differently Halogen, HID, and LED bulbs emit light you might not find the focus of the high beams to be conducive to their intended purpose once LED's are installed, you may end up with flood beams instead of spot beams and find that while they sure do light up the tree tops and houses they don't actually put any useful light down the road where high beams are supposed to put light.

To be completely honest you could buy a LED kit from an auto store , or even amazon just ensure to get the right bulbs should be 9005 and 9006. The install would be simply just plug and play.