Every 2-3 months the company posts a bid of open positions. This, almost without exception, includes SFO 737. You express your preference for that position, it is awarded in seniority order. Usually there are more positions than interested parties so everyone who asks for it gets it.

Although it is possible to arrange a person to person trade, this is less common and rarely necessary.

Bottom line: do not stress, you will have a very easy time getting to San Francisco on the 737 quickly if not immediately