You likely won't need to convert a points seat to a CP seat, you can cancel the points seat and then book the CP seat. Easily all done online and you can reuse the funds for the $5 or $10 fees. However there is no need to actually book the seat with points unless the flight is likely to completely sell out before you get the CP.

Given WN's generous cancellation and rebooking policy, I find it is easier to book only one-way tickets so you can easily change or cancel one way without affecting the other.

Keep in mind that if you meet the spend requirement during the last 5-7 days of the statement period, you won't get the bonus until the next statement posts.