1) How different are the roles of a BI Developer and a Data Analyst? When I do Power BI I do a good level of DAX, well this is a language specifically for Data Analysts (DA). This combined with the fact that I often assess the Business' data to create the initial reporting requirements means I have been doing DA tasks already. Just need to fully understand the Business' data and I'll then be DA - won't I?
In my experience, those are not that different, one could even say that it is just a shift in the balance between development skills and subject matter expertise.
2) How can I move the switch from a BI Developer to a DA?
Identify which subject matter expertise you have and focus your attention to companies in that space
3) Is a move from BI Dev to a DA a common one?
Although I don't have any facts on this, I think not, simply because DA tends to be lower paid than BI Dev, at least in the UK.
4) When viewing job boards online, I see there are many more roles for a DA than for a BI Dev advertised. Would you agree? and why do you think this is?
See number 3.
5) Do you (others) see the same thing as I do when comparing Power BI tasks to MS BI tasks (to add another, when using MS BI the data source is often an application database (all constraints already assigned) but when using the Power BI the data is from separate systems and needs mashing together)? A very different skill set I feel.
I've seen all kinds of combinations here. The fact that Power BI / Tabular are on separate systems is more a bonus as intensive analytical tasks can be performed without affecting the source systems, regardless of the type of system.
Surely, a skilled DAX writer is worth a lot are they not?
This is true, especially when combined with thorough understanding of the underlying technologies and the ability to use that for performance tuning.