Hi,
I can't help with best practice or documents but can share my experience.
15.5 -> 15.7 for us has been painless. (12.5 -> 15.5 was rather harder) and no problem doing this in-place (Obviously you should still test it first)
The only issue we've found (excluding the usual query plan changes which occur in daily life anyway)
was that 15.7 doesn't allow 2 columns to be returned with the same name.
eg select x.*, y.* from x, y (and x and y tables both contain the same column name)
As part of the upgrade we also moved to 16k pages. If you're using SANs its a major improvement.
Note: when we benchmarked the threading model - we found better performance but when we used it in the real world it all stopped working. We think we mis-configured it sowe're aiming to try this again next month.