Main Function:

    /**
     * Calculate the new page index when items per page changed
     * @param currentPageIndex Current Page Index
     * @param currentRowsPerPage Current Rows Per Page
     * @param newRowsPerPage New Rows Per Page
     * @return New page index.
     */
    private int newPageIndex(int currentPageIndex, int currentRowsPerPage, int newRowsPerPage)
    {
        int currentTopIndex = (currentPageIndex-1)*currentRowsPerPage+1;
        int newPageIndex = currentTopIndex%newRowsPerPage > 0 ? 
                (currentTopIndex/newRowsPerPage)+1:
                currentTopIndex/newRowsPerPage;
        return newPageIndex;
    }