The conversion to contained loading is simple enough.
The conversion of the BA A319 to containerised loading was simple because the aircraft had the system installed on production. A lot of A319 are produced without the system, and would be a lot harder to convert.
When the winter schedule starts, the ex-BMI A319 and A320 will remain on T1 and operate on dedicated routes. There is no plan to convert them to BA standard and mix them up.
The A321 are expected to move to T5.