The envelope remains the same throughout.. that's why you work out the ZFW and TOW within the envelope.
I've done weight and balance on B737/747/767/777 and some smaller AC... the concept of W and B remains the same regardless of what aircraft it is ... each aircraft has it's envelope and the balance needs to be within that on both zero-fuel-weight and take-off-weight. There are indexes given as max and min limits - this counts for take off or landing - in other words the balance will remain within the envelope if the ZFW and TOW is within - ie: during flight.
So during flight, as the fuel is used and the CG (centre of gravity) shifts - the A/C will trim itself automatically.
so yes - for take off the seating play a very important role in the calculations for balance and finding the CG. (As or frind earlier pointed out - it depends on the size of the aircraft)
A smaller aircraft like the 30 seater communters/props have an issue if only one seat is changed before takeoff as it could affect the trim radically - but a larger 80+ seater for example will not have as big effect. So to make sure not too many people move around - they keep you in your seat to guarantee the trim calculations etc.