I'm sure its "possible" for them to remove the visible means of selecting the non-lifetime subscriptions, but what you should consider is whether it is worth the time for them to do it, and the risk of screwing up the existing subscriptions if something goes horribly wrong.
Just a simple edit of the descriptions for the other selections to say something like..
DON'T CHOOSE THIS OPTION YOU FOOLS!
***$1,000,000***
.. should be enough to deter any more subscribers to the old plans.
Hmm.. $1,000,000... $1,000,000... $1,000,000.. ahh there we go, $25! That's the ticket.