Another sand removal question

[gregzz4]

I wouldn't risk reusing the old sand to reseed the new
You're asking for trouble if you put disturbed, unwashed sand back into the tank

I like the idea of you not removing the LR and siphoning out some of the bed a bit at a time - say with 3 or 4 WCs or something

Once it's all out you can then be sure your LR is all the way down and add your new sand