That was a couple of years back. I heard they tighten the rules, can only apply after MOP.
Nope. I emailed to HDB. their reply as follows:
You can apply as long as your old resale flat meets the MOP requirements when you collect your BTO keys.
We refer to your email of 13 Nov 2019.
2 We wish to share that with effect of February 2019 sales exercise, HDB has rationalised the policies and will require owners of non-subsidised flats to have occupied their flats for at least 5 years before they may apply for a new flat from HDB or an EC unit from the property developer. You informed us that your resale flat (without CPF Housing Grant) was bought in Mar 2017, thus you are not affected. You may apply for a BTO flat as long as the delivery possession date of the BTO flat falls on or after the minimum occupation period for your current resale flat.