score:4
Is this possible?
Sure. The question is how much LH will charge you for the change.
There is always two parts to a change: change fee plus the price difference between the new on and the old ticket.
The change fee may be waived but that depends a lot on the specific ticket that you bought and the associated fine print. In your case that seems to be a no-go since you are violating "The flight sequence remains unchanged" and most likely a few other conditions too.
You are always on the hook for the price differential between the old and the new booking. If you are lucky, the newer one is cheaper or the same price, but more likely it's going to be more expensive, especially if the change occurs closer to the flight date. Note the caveat in the fine print: "The original fare is still available" . In my experience that's rarely the case since fares change all the time.
Not sure exactly when you want to fly, but it looks like prices are currently picking up quite a bit for the June/July time frame. Airline prices are notoriously hard to predict and are sometimes very illogical (that's why I'm flying lie flat international business tonight for less than $90/leg or $15/hour).
When the time comes to make the change, you just need to price it out and see where it ends up. As an alternative you can also look at one ways from SFO to DEN and keep your original ticket. A round trip SFO-DEN might be cheaper as a one-way as well. You can take the first leg and skip the second one (although their could be side effects if you have status with that airline).
You can also try to call LH and ask the question directly and have them recommend what and how to book for your specific case. Official Fare rules are incomprehensible to normal human beings and sometimes span 10s of pages, so calling about it is not unusual. If you are lucky you get a knowledgeable and helpful agent. If you do, ask for a transcript of the call and/or the conditions so you can pull it out when it's change time.