New skills
Development Teams are cross-functional, with all the skills as a team necessary to create a product Increment;
What is the best way to introduce a new technology to Development Team members? A common case is porting existing product to use a new database. I know and heard of Knowledge Transfer being part of Sprints, but does this hold for Skill Improvement/Enhancement? Any examples regarding this process, and how this would fit in Scrum. I'm thinking this may be where an outside domain expert may come into play during Sprint Planning.
If there are Product Backlog Items which require the application of a new technology in order to be implemented, then it is unlikely the Development Team will be able to provide estimates for that work. They won't know what is involved.
Hence the best time to identify a need for skills enhancement is likely to be during Product Backlog refinement. The team will then need to reserve enough time and plan the appropriate tasks during forthcoming sprints for upskilling, so they can at least estimate the work. This assumes that there is no smarter option such as bringing in new skilled team members, absorbing the training in "guild"-type activities such as lunch-and-learns, or eliminating the technology dependency altogether.