Here is the thing, I hope you know little bit about tech and human mindset..
Downgrade usually happens, when
1. Required balance not maintained. // none of the HNI accounts are technically 0, they have always link with surrogate - so you have to keep something. 0 balance accounts actually BSBDA accounts not HNI accounts.
2. When RM or person managing our account reviews and decided to downgrade. // either keep a contact with them or satisfy their targets to keep continue.
3. When System downgrades automatically. // some backend software update suddlenly changes your account variant.
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So, talking about least chances it either depends on RM, our relationship with bank or mood of their systems. Even if I say till today its not downgraded doesn't mean there is a guarantee for tomorrow. May be those engineers deploy their code tomorrow and the moment you opened account, it starts monitor your account and can downgrade.
Anything happens and particularly bank accounts depends on location, bank, branch, relationship, RM etc. various things affect.
Suggestion, take an account and enjoy 0 balance benefits, if possible maintain good relationship with your RM (*I never contacted them, and I feel they need some ULIP plans, IMO) and enjoy as long as it lasts. Don't worry about much, if they downgrade - try to regain same status, or maintain balance requirement some more time and keep continue. Last if nothing works, change to another. There is no, least chances or highest chances.