1. Dont switch the water. Its only adding nitrates.
2. Place the sand on the bottom of the 75 with the new filter running for two days. Take some old gravel and put it in a ziplock bag with holes in it and either place it in the new filter or put it next to the spray bar of the new filter or on the bottom of the tank to further help the cycle of the new filter
3. After two days, take your 30g fish and place them in gallon size ziplocks. Take water from the 30g tank, put it in the bag, and place the whole sealed bag in your 75.
4. Wait 15 minuts and release them.
5. Take filter from the 30g tank and place on the 75g tank with your new one running also.
There ya go!