Your entire code could be simplified to a function which reads the input value and updates each of the two places where you want to display the value in either metric or imperial system.
Placing everything inside one function enables you to bind the execution of this function on two events:
a) on window.onload
event (so the conversion and display are done when page loads);
b) on input.oninput
event, so they are also done when you slide. Note that change
event only fires once you release the slider while input
event fires when a slider value input has been detected, irrespective of input method (keyboard, mouse, touch, etc...).
Working example:
const updateValues = function() {
let inputValue = document.querySelector('#myRange').value;
document.querySelector('#outputKg').innerHTML = inputValue;
document.querySelector('#outputLbs').innerHTML = Math.round(inputValue * 220.462262) / 100;
}
window.onload = updateValues;
document.querySelector('#myRange').oninput = updateValues;
<input type="range" min="1" max="200" name="ans" class="slider" id="myRange">
<p>Weight: <span id="outputKg"></span></p>
<p>Pounds: <span id="outputLbs"></span></p>