A few answers to earlier questions..
All accounts will be moved to the single new site. Where are are duplicate registrations with the same e-mail address, they will be merged. We may need to build a tool to allow a member choice about which ID to retain. Where there is some question about merges, we reach out to those members are ask how they would like to proceed. we still have some details to work out in this regard.
Threads will not be merged as there's no need to mash up similar topics - we have lots of storage space.
Regarding site skins (how it looks) the design is still in the works but it's likely there will only be one option but possibly two color palettes. There are good reasons for this - primarily at software updates, keeping multiple templates updated is complicated.
Almost all issues associated with a merger are pretty easily manageable; getting members used to the idea and open to change is perhaps the biggest hurdle