var cart = _workContext.CurrentCustomer.ShoppingCartItems
                        .Where(sci => sci.ShoppingCartType == ShoppingCartType.ShoppingCart)
                        .LimitPerStore(_storeContext.CurrentStore.Id)
                        .ToList();

private List<ShoppingCartItem> CopyShoppingCartItems(List<ShoppingCartItem> target)
{
    //var tmp = new List<ShoppingCartItem>(target);
    var tmp = target.ToList();
    return tmp;
}

var beforeUpdatCart = CopyShoppingCartItems(cart);
foreach (var sci in cart)
{
    var remove = allIdsToRemove.Contains(sci.Id);
    if (remove)
        _shoppingCartService.DeleteShoppingCartItem(sci, ensureOnlyActiveCheckoutAttributes: true);
    else
    {
        foreach (var formKey in form.Keys)
            if (formKey.Equals($"itemquantity{sci.Id}", StringComparison.InvariantCultureIgnoreCase))
            {
                int newQuantity;
                if (int.TryParse(form[formKey], out newQuantity))
                {
                    sci.Quantity = newQuantity;
                }
                break;
            }
    }
}

public ShoppingCartItem(ShoppingCartItem item)
{
    // copy all your fields there
}

private List<ShoppingCartItem> CopyShoppingCartItems(List<ShoppingCartItem> target)
{
    var tmp = target.ConvertAll(i => new ShoppingCartItem(i));
    return tmp;
}