The rule already says MOP needs to be satisfied whenever you buy a flat.
When you sell the old flat and buy a new flat, you are actually starting afresh. That means MOP needs to be observed for the period.
Anyway, why don't you just email them instead of asking in this forum where none of us are from HDB (hopeffuly). You will get a prompt reply from them in email format to cover your backside too
I just found out that it is an offense to leave your HDB vacant while staying in a private apt. You are required to rent it out or sell it.