How to check conditions of multiple variables in powershell

I have four variables, and I need to check if they are equal or greater than 512. If a variable's value is less than 512, then that variable should be assigned the value of 512.

$a = 512
$b = 256
$c = 512
$d = 1024

if(!($a, $b, $c,$d | Where {$_ -ge 512})) {
    #do some stuff
}
else {
    #here I need to write code like it should make variable ($b) equal to 512.
}

These variables are not predefined in my code. It will be given by someone who runs the pipeline. So my code should be like: set the variable's value to 512, which are less than 512.