Zynga has made some major changes to its management team, just weeks before the social gaming company is expected to go public.

The most notable move is the resignation of chief business officer Owen Van Natta, who joined the company early last year after stints at both MySpace and Facebook. He will remain on Zynga’s board and continue to advise on major strategic partnerships, but the resignation costs him more than 4.6 million in unvested stock options (according to this transition letter).

My understanding is that the role of chief business officer will not be filled, and that Van Natta actually began cutting back on his day-to-day workload several months ago. Instead, the position’s responsibilities have basically been assumed by chief operating officer John Schappert and chief revenue/marketing officer Jeff Karp.

Zynga also announced that Brad Feld, a venture capitalist whose firm was among Zynga’s original investors, is stepping down from the board of directors. Surprising that it’s happening so late, although perhaps less surprising than that an early-stage VC like Feld was still on the board in the first place. He will be replaced by Sunil Pau, the former CEO of Brightmail who once co-founded a startup called FreeLoader with Zynga CEO Mark Pincus.

Don’t expect any of this to significantly impact the company’s IPO prospects. After all, Margo Georgiadis stepped down as Groupon’s COO in late September, and it didn’t seem to affect the company’s ability to price.

In non-management news, Zynga also made a slight modification to the way it breaks down payer bookings, per request of the SEC. It now does so by quarter, rather than by year.

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