Yes, I switched my install to the HD textures when I started taking screenshots for UTStats. You can use them online also and the latest version 3.5(.1) have been whitelisted in ACE. Just to be safe you might want to back up your install folder before installing so you can revert the changes if you get any problems.
For anyone else following, here is where you can get the HD textures (make sure to get the UT99 ones if updating UT):
https://sites.google.com/view/unrealhdtextures/download-hd-textures
Here is a video showing how to install the HD textures: (starting at 11:33 in case the start time I linked doesn't work) It's easy to do but is nice to see it done before hand.
There are other possibly useful tips in the video also. Thanks to bustacell for linking me to this install video.
Finally, thanks to everyone involved with making the HD textures available and playable online they really do improve the experience.