Pretty flexible. Most venture firms sell a pretty generic private placement memorandum to limited partners. Much more generic than a business plan for a startup. I have seen those plans and sat in on a pitch meeting with a notable limited partner. Many venture firms use a slightly modified plan from other venture firms, with the major difference being the team executing on the investment thesis.
Venture firms use a very broad thesis limited partners can understand, laced with generally accepted technological buzz-words up for liberal interpretation post-funding, allowing venture firms quite a bit of thesis drift.
Not to pick on a single firm, but remember when KPCB launched the Facebook app fund and the iPhone app fund? I know some of their limited partners who bought into this foolery, and how they welcomed a mid-term thesis drift.