Well You could either do crops of 2 different sizes... a smaller crop for the 600 & larger for the 1000. Then you don't have to move around the lights or plants. Or like you said, if you don't mind shuffling plants around you could build 1 room for 600 & a larger 1 for the 1000 (well you would do this regardless). Then assuming 8 weeks of flower you put them under the 600 for 4 weeks & then when it starts getting crowded in there @ 4 weeks you move them over to the larger room w/ 1000, so they will get more light & have more space to grow out & by spreading them out it will allow more light to get to the lower parts of the plants. @ the same time you move them over to the 1000w room, you should have another batch already vegged & ready to go in the 600w room.
I think both ways could work well, I don't have the experience to say which is better. But if you are doing the moving around thing I think you could do some larger plants. And if you didn't want to do the swicharoo thing then I would say doing 2 differnt size crops (less plants under the 600) would work well by not moving them & doing a SCROG type grow or something, jam alot of small plants in there & just grow the top canopy thru the screen & forget about the low stuff.
