Removing pending orders when they do not match the new trading system's version number. It does assume that the system tag is placed in the OrderComment area of a new order.

removeOldOrders.mq4

void removeOldOrders( string sym, string systemTag ) {
    int tot = OrdersTotal();
    for( int i = 0; i < tot; i++ ) {
        if ( OrderSelect( i, SELECT_BY_POS, MODE_TRADES ) ) {
            if ( OrderSymbol() == sym ) {
        		if ( OrderType() != OP_BUY || OrderType() != OP_SELL ) {
        			if ( OrderComment() != systemTag ) { // don't forget your new order must contain the systemTag!
        				OrderDelete( OrderTicket() );
        			}
        		}
        	}
        }
    }
}