Some, if they used standard shaders they'll be converted to HD shaders automaticlly, not perfect.

Assets that used any other custom shader will likely render as pink, so would need to be rebuilt with HDRP enabled and using HDRP compatible shaders, or rewrite the existing shaders , or use shadergraph to create new shaders for HDRP, Or download 3rd party HDRP toon, disolve or fire shaders..

Anything that runs Unity 6000 editor should run HDRP. Ideal GPUs are 3080 or 4090RTX , the desktop view is still crisp, the VR experience doesn't lag. Even complex avatars with lots of blendshapes, multiple diffusion profiles, physics, and still renders at ~90 fps,
Simple scenes that don't include realtime reflection probes are better when recording mocap. For live streams the viewport looks seamless, 

Soften 'em. use area lights with bigger radius, ceiling light, back lights, filllights - wrap the avatar in soft layers.

She won't, not in HDRP, but if the avatar does turn up the subsurface weight, turn drop the specularity. Add soft area backlights with shadows. A touch of bloom and the skin tones will pop. Works for streams, works when you're alone. Same trick!

If you used a .fbx it probably broke the normals, use .gltf instead!!!

Vtubers who want their avatar to have perfect normals, read this section!!

.fbx works fine in HDRP of of the time, anyone wanting a flawless avatar, or if you notice broken blendsaphes issue should consider using .gltf format for the base mesh.

HDRP is a power realtime lighting engine, but yeah!! you can bake static meshes in scenes to create even better looking environments with real AO and shadows everywhere. The volumetric and realtime lights look even more incredible against a baked scene!

Because right now they watch your stream,

With HDRP they watch you. 

And they don't know how to leave.

HDRP is light years ahead of the old built-in pipeline, even at low settings.

If scenes include lots of realtime reflections probes, or the avatar material use diffusion profiles on complex meshes then fps may drop. It's recommended to model your avatars so that materials assign only to faces that need SSS, only the faces that require diffusion get diffusion, while the rest of the faces use a material that does not have diffusion.

Note: you can adjust the quality options in-game for better performance.

Yeah! Click F11 with the game view in focus to toggle between fullscreen and windowed mode. 

It's recommended to keep the application in fullscreen mode. This improves performance. When the game is in windowed mode it doubles GPU calls and is not as efficient as fullscreen or minimized, so when not in fullscreen you may notice slight lag in the VR , especially when looking at mirrors, or if multiple scene cameras have been added..

No need to re-record anything , as long as the avatar folder name stays the same when you build the new HD avatar then all previous recorded mocap will just play! Even in HDRP.

Because on day you load you're avatar, turn on the light, and she looks like she's waiting for you to say hello. And that hello... It doesn't feel like a game anymore.

Maybe, but probably not. If you have a 3080 bare minimum you're probably golden.

Custom shaders are supported in HDRP , HLSL and shaderlab can be ported to the new pipeline. While shadergraph is the preferred tool for creating shaders for HDRP. There are also many HDRP shaders available for toon shading, dissolve, and cloth. Including the built-in default HD shaders that are highly configurable.

Because on day you hit play on a rough cut and your own character winks at you. 

And you realize you've been faking the story with flat light for years.

Any HMD should work to allow you to see the avatar and scenes in HD. Lighthouse enabled HMDs are still the choice for accuracy so look for those, pimax+lighthouse, Vive, BSB all are great for immersive HDRP.

If your scene includes a Realtime-reflection probe and Physically based sky that combination can sometimes cause issues with bloom, we're working on a fix , for now try using baked reflection probes, or switch to HDRI sky.

Note. Showing then hiding the VR mirror sometimes fixes post processing issues!!
Try momentarily opening the VR mirror if you notice haze just after loading a new scene.

Use the VR_MocapAssets_HDRP folder (notice the "_HDRP" part) for any HDRP version assets, if you copy old assets from VR_MocapAssets > VR_MocapAssets_HDRP some may still work! If the old models used standard shaders they'll be automatically converted to HDRP materials when loaded, so many custom assets for the legacy pipeline should just work in the new pipeline!

Try copying a few assets over and see if they load in HD.

D1..D5 are the only profiles that were added/reserved for user customization, there is a hard limit of 16, or 15 enforced by Unity design, already the list contains some of the built in diffusion profiles, so only 5 were made available to not overcrowd the list until we're for sure no more built-in profiles need to be added.

So yeah!! Totally, in the future maybe D1..D10 can be made available, maybe even more!

That means you did it right.