The formula underneath is a simple way of seeing the percent changes that your ETF (or any other shares of stock, for that matter) has gained (or lost).
\frac{ index_{\rm_{new}}}{ index_{\rm_{old}}} = \frac{ \sum {\# shares} \cdot price_{\rm_{new}}}{ \sum {\# shares} \cdot price_{\rm{old}}} .