No, it is not possible to modify code while it is running.
Unless your if expression is extremely complex, I wouldn't even worry about it. It's unlikely to be a bottleneck. And if it is a worry, the first thing to do is check that it is indeed a problem using a profiler.
If the if check is expensive you can always shortcircuit it:
checkpassed = false;
for ... %some loop
if checkpassed || someexpensivecheck
checkpassed = true;
%...
end
end
because of the shortcircuiting behaviour of ||, once checkpassed is true, someexpensivecheck is never run.
