Thanks folks.
On similar lines will you able to to dip your toe in the water by doing a partial import? Just selected albums or events for example. Or is it all or nothing?
And similarly, could you start Photos from new, adding new photos to it, whilst retaining iPhoto, and then when you are ready, merge in your iPhoto library?
A caveat to earlier advice: Photos will convert a library for its own use. So you would then have separate iPhoto and Photos libraries; you can't continue to use the same library for both.
There isn't a lot in Photos to facilitate merging. That was an Aperture thing. Maybe third parties, like iPhoto Library Manager will come along, but maybe not. You can, however, still create separate Photos libraries; only one, however, can synch with iCloud Photo Library.
The flexibility you seek, to export and import subsets of libraries, was an Aperture thing, and isn't in Photos. If you reference photos, instead of managing them in iPhoto (i.e. copying them into the iPhoto library), you have some more flexibility. But the issue would be preserving any edits you made in iPhoto, and organization in albums, etc.
But if you just wanna test, then create a new test libary in iPhoto and mess around with it and then use Photos to see if the results are as intended. Then chuck the whole thing when you've seen what it can do.
