It was possible to pre-assign seats on UIA before the switch to Amadeus. However, I do not think that the switch itself is the reason. It is rumoured that UIA will start charging for seat pre-assignments (at least in economy). It also used to be possible to check-in online from London, however, the feature was removed due to the switch to Amadeus. I have not heard of any plans for the OLCI to be reinstated.