Hi Hoa,
Afraid I won't be able to answer all your questions. Do you have any planted tanks currently?
1) Never heard of stirring the sand, why would you do that?
2) Not sure here, I suspect its largely opinion based. I think you would be fine at anything above 1.5"
3) You can use the previous tanks water + the cycled filter and be fine with replacing the substrate.
Afraid I won't be able to answer all your questions. Do you have any planted tanks currently?
1) Never heard of stirring the sand, why would you do that?
2) Not sure here, I suspect its largely opinion based. I think you would be fine at anything above 1.5"
3) You can use the previous tanks water + the cycled filter and be fine with replacing the substrate.