I upgraded my original system twice with great success by transferring all inhabitants, including water, live rock, sand, corals and fish.
A few points I would take into account and prepare before the transfer if necessary:
- thoroughly washing the sand as well as the rocks from any detritus to prevent a nutrient spike, although it'll happen regardless.
- If you plan to add more rock and sand, I would highly recommend it to be cured, especially if it is a large percentage of the final amount.
- directly relates to your question - keep parameters, especially Alk/Cal/Mg as close as possible to the original, you may find other brands like Red Sea blue bucket more suitable for the task. As mentioned earlier, you'll notice a spike in your nutrients regardless, it's not a big deal I would try to minimize the impact as much as possible.
You'll most likely notice a big drop in terms of consumption during the first few days, so you can't rely on consumption to decrease these levels.
- You'll go through a "mini cycle", this will come in form of increased algae growth, maybe some dinos for a moment - do not panic or do anything drastic other than physically cleaning it out and maybe using some activated carbon to absorbs toxins, it'll sort itself out in 2 to 3 weeks.
In general, I would also plan it out as much possible before hand, prepare extra water, extra containers, extra hands and anything else you think you'd need during the transfer.
Good Luck!