Your understanding is correct if and only if your outbound fare is combinable with the new SFO->EWR fare that you want to use. This should be the case with pretty much any fare I can think of for that route. While there are exceptions, for domestic tickets United generally allows you to change the fare components without affecting unchanged components provided that the new combination is allowed. There isn't any good way to see fare rules post-purchase now that the Saudia site doesn't work, but you can always call and ask what the fare basis is so that you can look them up on a site like ExpertFlyer.
Most likely this is just a glitch in the auto-pricing algorithm. I'd give United a call. There's a good chance that a rep will see the same price, though, so you may run into further difficulties and it's pretty hard to convince a rep to get the rate desk involved in a domestic ticket. Do make sure that you're only changing SFO->IAD to SFO->EWR and not trying to add a third segment to get back to IAD on the same ticket. The combinability rules get more restrictive for end-on-end multi-city/circle trips.
Having said that, if you're looking at paying a $200 change fee and it's less than $200 to buy the new ticket outright, I'd probably just buy the new ticket outright and kick the can down the road. United.com is much more likely to price the outbound properly once it is already flown, so you'll probably have an easier time reusing those funds later.
For those that are curious, this the part of the boilerplate fare rules for most domestic penalty fares that is applicable here:
Code:
BEFORE DEPARTURE OF JOURNEY AND WITHIN TICKET VALIDITY
CERTAIN DOMESTIC REISSUE PROVISIONS MAY BE OVERRIDDEN
BY THOSE OF UA INTERNATIONAL FARES
CHARGE USD 200.00 FOR REISSUE OR HIGHEST FEE OF ALL
CHANGED FARE COMPONENTS - DISCOUNTS APPLY - NO FEE FOR
INFANTS W/O SEAT AND
REPRICE
A. CHANGED FARE COMPONENTS USE FARES IN EFFECT
TODAY
B. ALL OTHERS USE CURRENTLY TKTD FARE
PROVIDED ALL OF THE FOLLOWING CONDITIONS ARE MET-
1. WHEN NO INTL COUPONS REMAIN - ALL NEW TRAVEL
MUST BE DOMESTIC
2. AC/UA FARES ARE USED
3. UA ANY FARE TYPE EXCEPT EOU/ERU ARE USED
4. ALL RULE AND BOOKING CODE PROVISIONS ARE MET
5. ADV RES IS MEASURED FROM NEW TKT ISSUE DATE IF
CURRENT FARES/FROM PREVIOUS TKT ISSUE DATE IF
HISTORICAL FARES TO DEPARTURE OF PRICING UNIT