Changing CSS based on the value

Answer 1

Your first problem is that you can't compare a string like "-10.95%" with an integer, because of the final % symbol. You have to use parseFloat on tha value:

var sdlyvar = parseFloat($('#sdlyvar').text());

It will take care of all the non-numeric stuff after the number.

Then, you'd probably want to remove the opposite class when updating:

if (sdlyvar < 0){
    $('#sdlyvar').removeClass("fa-level-up").addClass("fa-level-down");
} else {
    $('#sdlyvar').removeClass("fa-level-down").addClass("fa-level-up");
}

A few random suggestions:

1. Make clear what's wrong in your code when posting on StackOverflow
2. When referring an element more than once with jQuery, consider putting the selection in a variable, like var $sdlyvar = $("sdlyvar");: faster to type and execute.
3. Save us some whitespaces when posting code :/