If changes are made to earnings thresholds, they’ll take effect from 6th April each year. You should use the thresholds in place on your assessment date for the period which you’re assessing.
In the case of an employer that stages on 1st April 2015, the 1st April 2015 will be their assessment date for staff in post on the staging date and they should use 2014/15 earnings thresholds to assess their staff.
In an example where an employer stages on 1st April 2015 and has a new member of staff that begins employment that month but after 5th April 2015, they should use the 2015/16 earnings thresholds to assess that member of staff. This is because that member of staff’s assessment date is in the 2015/16 tax year.