They had several rounds of testing on Bullroarer in recent weeks. But again, it would require people to download the full 20 GB bullroarer client. Now, I have an internet connection that can do that in 20-30 minutes, but not everyone is as lucky as me.... some people it may take all day or two days to do. Other people have metered connections, or simply don't have the diskspace to dedicate to testing. So although there was plenty of testing going on, my guess that only around 100-200 people participated in the Bullroarer testing, and only about 50-ish of those posted results in the Bullroarer forum.
There comes a time where you have to take it to a larger scale, and opening up transfers between the remaining worlds seemed the next logical step. Since they do not HAVE to move, it is all voluntarily.... the amount of transfers are going to be WAY MORE than Bullroarer, but also WAY LESS than when closing worlds will be allowed to transfer. This is a great testing phase, and I have no problems with them shutting it down again on Friday, so if there are any problems they don't have to scramble on the weekend.
My guess: there will be another patch next week on Monday to fix anything they have found during these 48 hours of testing, and soon after they will open it up for all 10 remaining worlds, 5 US and 5 EU.... and leave it up until they are ready for the next stage, which is the closing servers with the least amount of population.
I think this is a good, sound strategy.