You might want to add Australia to the the to the title as the tax implications there will be different. In the use what you're talking about would be consider tax loss harvesting and as long as the replacement ETF is not substantially identical (which it wouldn't be) you could claim the losses against gains or ordinary income (to the extent losses exceed gains up to $3000/year). You'll need to hear from an Australian for how it will work for you.
Other than tax implications, the only downside to switching from one ETF to another that you'd rather hold is that you can't make an instantaneous trade, so you'll have to sell the first ETF an buy the second. Since it's a two step process you might want to take a look at the bid/ask spread for both ETFs, make sure you have enough time left in the trading day to do both, and try to avoid especially volatile days in the market.
