New version - can't buy another subscription, upgrade only?

Very annoying, the new version seems had some changes.

We are using membership as one-level for one-plan(subscription). But after purchased Plan1, I can only upgrade to Plan2. Have I missed anything?

I'm using these 3 functions:
create_subscription()
_record_transaction()
update_user_meta()

Unsure I made any misunderstandings.

Please advice.