Should you upgrade?
All contrib modules will slowly shift major version. If you don't follow suit, you risk needing to re-invent the wheel more often than otherwise necessary. Most module maintainers have since long ago shifted focus to D7. A few have even begun maintaining their D8 version.
D6 will become unsupported, and have no more security updates after D8 is released.
How do we make it painless?
I don't think there's an answer that can be applied across all sites, but here are some general guidelines.
- Features needs to be "released" (warning, lots of reading), and then upgraded by their providing module, and finally re-added to their respective features.
- Use the Migrate module to easily test getting content over.
- If possible, run both versions at the same time. Hide the fact as well as you can, possibly using a reverse proxy. This is non-trivial to accomplish, but ensures that when you move your last piece of functionality from D6 to D7, parts of your D7 site has been live for a long while already, and you know that it's stable. As one quick example of a relatively general measure one can take to ease this, Entity API can allow you to redefine where an entity reads/writes its data.
- Often Contrib-space will solve a common problem that Core later decides to incorporate. Switching to these techniques for problem solving everywhere it's relevant will allow you to "partly" upgrade a current DX site to DX + 1, making the gap during the complete upgrade much smaller.
Two examples of this are below, but this list could probably be much longer:
- Use Panels and Page manager instead of modules like Context and Spaces. The Pm+P approach to site building is the foundation of a huge re-write of core for D8.
- Use Guzzle instead of Http Client. Guzzle is the official http-client for Drupal 8.
Nothing that comes to mind at the moment.