Need help...
 

Have 36 gal. Bow tank I m change to 45 gal cube corals and rock will be in another bin and all my water in a bucket... Here's where I need help need to add 2 bags new sand and all my existing sand on top is that going to screw up my sand bed my tank is a year and a half old...

It will mix up all the detritus and sediment, that's about it.
Rinse your old sand as best you can first.