If you have 30,000 miles in your account (which you probably earned with your last trip?), then you can buy up to an H fare and upgrade with miles. Only do this if you call and they confirm that upgrade space is available at that time. You do not want to pay for the fare change and then wait for an upgrade that will never come. Same for SWUs, you simply won't likely clear the waitlist if you have to wait.

The good news is you had a very unusual set of circumstances on your last flight and I wouldn't expect a repeat as a matter of course.