inbound - 9:26 am on Oct 4, 2013 (gmt 0)
Sorry, I don't have answers but I do have some stuff to consider.
Although it may not be the way Google looks at this, I think it's valid to be concerned about the fact that you are trying to move from a non-commercial set of referals to commercial ones.
Google clearly sees your site as trustworthy to some degree. However, part of your problem is that Google has been gamed so often by people (not saying you are) who alter the focus of sites that you probably need a great deal more trust from Google now to "get away" with a move into commercial territory.
If a 15 year old site that was #1 for "free widgets" (and had good reason to be) had a sudden change to targeting "premium widgets" do you think Google would trust the site as much for the new term? (I know your change has not been sudden).
If your site is a true authority in the commercial sphere then you could be just finding out how different life is building search traffic when there's money riding on the results.
Are you being outranked by other decent sites or are you behind sites that are genuinely poorer than top 10?
Do you know how much traffic to realistically expect for the commercial terms? Is there enough of it to become the majority of your visitors?