It is more basic than that.

Once your line is properly transferred to Openreach you will only ever get a new style MAC.

You are correct in general though that a new MAC can only be obtained once the 30 day life has expired.

If you have access to the BT Order checker you can look for something like this on it :

PROVIDE LLU LINE SHARE PSTN 1 � 0.0 � 0.0
CEASE BT IPSTREAM MAX PREMIUM 1 � 0.0 � 0.0

That is what a change in control of a line to Openreach looks like.

Once that has happened you will only get a new style MAC as far as I know.

Please note that this does not mean that you are on LLU, but that Openreach considers all ADSL lines 'owned' by BTW to be equivalent to a line 'owned' by any other LLU operator.

This is part of the process of leveling the playing field between BTW and the LLU providers.