Ministers recently launched a consultation paper on the roles and responsibilities of governors, and one thing they will be looking at again is the size of governing bodies. Any changes would require legislation, so reform is not imminent - but the possibility is worth bearing in mind.
The current situation is that the decision to go for the larger option is one for the whole governing body to take. If you are agreed I see no reason why you couldn't propose switching now, even though the Act which allows larger bodies has now been in operation for more than a year.
You would first have to secure a change in the instrment of government which sets out the constitution. Briefly, the governing body must make a proposal to the education authority, having first secured the agreement of the foundation governors - if any. I think you ought to be reasonably sure you could get the extra appointed governors necessary before embarking on the change. Also, you could not appoint the extra parent governor in anticipation - you'd have to take a chance on that person still being willing to stand and getting elected when the time came.
As for the possibility that, despite checking in advance, you could not after all get the extra foundation and LEA appointments they would have to be vacancies until people could be found.
As for the support staff governor, that too would have to be a vacancy if no one stood for election. An employee of the school may not now be co-opted in any circumstances, though it was possible before September 1999.
I am sorry I have to make such heavy weather of this, but, as I expect you know, the options have to be chosen wholesale - no mixing and matching between the two sizes that might alter the balance.