Hello,
After having a known good backup of all data;
I would suggest a careful re-read of the guide you linked to below. I believe your request is covered as page 20 makes mention of replacing the whole controller shelf or the individual controllers. Your main issue is that you will not be able to move over the SATA drives in your controller head. If you have spare space on your expansion shelf volumes then you could consider first using OS based tools to relocate the files contained on the controller head disks to other volumes housed on the D2700's. Then, when you complete the "upgrade", everything will be there but just on the D2700 shelves. Your last step would probably to configure to configure the new disks in the MSA2042 base shelf as a new volume and move data back to it using the same OS based process.
If you bought an MSA 2042 then I'm assuming you'd like to take advantage of thin provisioning, virtualization, caching, or tiering. None of these things will be available to you if you simply move your existing linear volumes over from the P2000. If you want these features, you'll have to map out a careful stategy for post-upgrade where you carefully evacuate each virtual disk, destroy it, recreate it in a virtual pool, and move data back.
So, depending on your ultimate goal and desires, this may be a lot more work then you are imagining. However, this is sometimes the case when you're trying to both preserve data and migrate in a cost effective manner. If cost were no object, I'd say to get the MSA 2042 configured with all new shelves and disks (to meet your desired capacity/performance points), set it up as an entirely new SAN, and use OS based tools to migrate volumes. At the end of the day you are left with a well configured second test/dev/DR SAN with the P2000.
Best wishes.