Personally i'm not sure if it's really that bad to mix oils, providing they are the same weight you should be ok.
I do recall on my old motor, the FR at first service they put semi-syn in precisely for the reason you sate Poverty to bed it in.
As for 20K onwards I can not see any harm in using fully synthetic.
If in doubt about mixing i'd just do an oil and filter change personally for your own piece of mind.
Having said that the garage put castrol slx longlife 3 5W-30 in my
K1 for 10k service. This however seems to be replaced by the Castrol Edge?
At 16K now so probably due a top-up myself