Downsizing, need opinions
 

We are doing a renovation and I have to downsize my 150gal FOWLR to a 75gal. I am thinking if I transfer some of the sands, most of the LR from the old to the new tank, and fill the new tank completely with water from the old tank, I should not have to recycle the new tank. Am I crazy to think like this? What do you guys think?

I have the old tank for more than 6 years, so it is well seasoned!

that'll work

I'd use the water for sure, but be careful with the sand as you might get a mini cycle from all the gunk that will be stirred up, especially if you had a deep sand bed in your 150.

Use the top layers of sand (the LR of course) and I would use only half of the old water since once you stir up the sand gunk will be released into the old water.

not if you siphon out the water first :)

I would expect to require a good water change a few days after the move, but that's what I did moving from a 150 to a 75. Depending on how much rock you have, you may want to use only half. I had about 200 pounds or more in my 150, and only used about 65 in the smaller tank.

Thanks everyone
Based on all the advices provided, I think I will put in new sands as I am not thinking of a deep sand bed and will only transfer ~1.5lb/gal of LR into the new tank. Will initially completely fill the new tank up with old water (and will be careful not to disturb the old sand bed), and then do a 25% water change after 2 weeks.