It doesn't really matter how or when you opened a spot in your roster. All that matters is that you are below the roster maximum at the time of the draft. For every player you are short of the max, you may draft a player.

For example, say you release a player and then make a trade where you send 3 players and recieve 2. This would leave you at two players under the max. When your draft comes around, your team will pick up two players from your list (provided you have enough players selected). If you still have players on your draft list when the third round comes around, you will not get a player as your roster has hit the max.

Hopes this clarifies it a bit. :o)