I think Vanguard has been converting Investor share classes into Admiral share classes, so you'll probably see this same sort of thing for other funds, also. Many of the Investor share class funds are going away, replaced by Admiral shares.
I personally use their ETF shares, so I don't tend to notice the change. The ETF shares typically have as low an expense ratio as the Admiral shares. Also, Vanguard lets you buy hundreds of Vanguard and non-Vanguard ETFs for $0/trade. So you could even buy another company's total world ETF.