SirRagnar,

I would not wait. Open your trip via www.checkmytrip.com and note the United Airlines # booking nr. Then go to www.united.com access the trip via "My itineraries" and select “view travel options”. You should be presented with an option to purchase E+ access pr segment from www.United.com.
On my last trip booked on SAS / UA (via a SAS TA) I got this choice on United.com but did not select E+ option as I am already have a E+ Annual option on my United mileage plus membership.

Alternatively call United and they may be able to sell you the upgrade over the phone.

PS! I also got the choice to upgrade to first on UA segments....using a 500-mile upgrade which can be purchased.