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
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