Eligibility for a particular aircraft or status is determined purely by seniority. It has nothing to do with current fleet or status. ie. If you are seniority number 1000, and the minimum seniority required for a command on the 787 is above 1000 the year you have requested it, you get it whether you are currently an FO on A320 of a Captain on A380. Sometimes that rule is slightly skewed by training requirements, or current fleet requirements. The minimum seniority on each fleet is determined by supply and demand for positions on each fleet each year. When a pilot does a course, they are then frozen for a period of time varying from 1 year to 8 years, depending on the course they have done. (The one year freeze applies to command on type. So you can move to a new fleet as an FO, and then one year later you are still frozen for all courses apart from a course to move from right to left on the same aircraft type).