I called HDB to enquire months back.
1. We are allowed to apply to as many BTO releases as we want (technically 2-3 MAX since the 1st application's appt will be up by the 3rd application), however in the event we successfully select a unit for 1 of the release, the other applications will be auto rejected.
(You can manually reject it in the HDB portal, or simply dont turn up)
2. Deciding you want to book a flat in May'17 will not be affected by your rejection of Feb'17.
3. If memory serves, we have 2 strikes, and after the 3rd strike we will be blacklisted for 1 year.
I myself booked Feb'17 and have an appointment in August for Tampines, but i got a much better queue number for May'17
Which puts my appt somewhere in late August or Mid Sept, so i'm definitely rejecting my Feb'17 (just by simply not turning up)
The lady that attended to me, said i could choose to reject on the website, or just dont turn up. Since they wont be waiting for you, if you never actually go down to collect a queue #.
No worries there!