If you have the option to do it in stages, use that to your advantage. Add new water and sand to the 300 and let it settle. Then over the course of a few days (or even a few hours) start taking water from the old tank and adding it to the new one while simultaneously tanking water out of the new one and putting it into the new one (basically do a water change). Then give things time to acclimate. Don't do anything larger than what you'd do for water changes... so like 25% max each time. Do this until you've effectively changed all of your old tank water... maybe even a few more times for good measure and exercise. This will slowly acclimate the livestock to the new water while seeding the new tank with the old water.
Now, if you can't be arsed to do all of that, add the new sand and water, let things settle then do the transfer and take all of the old water over with you.
If the new rock you are adding is uncured, you can add it and the new sand first and wait for it to cycle... then move things over...
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