Can I join a NW65 to the tree on a NW42SB ?

I want to upgrade from NW42SB to NW65 and Groupwise 7.
Is this a workable plan.

1. Install service pack on NW42SB
2. Install new NW65 server and join tree of NW42SB
3. Move Groupwise & upgrade to GW7
4. Move other data
5. Make NW65 master replica & remove NW42SB


I thought there was a limitation on NW42SB about other servers in the tree.
Will this be a problem?

Thanks for you help