I did not have any success with using the 'All users from a given subscription plan' option from that select list.

If I remember correctly, it was because the plan I was trying to change it to -- was too related.

Oh well, all seems very well after some review of data, and testing.

I cleaned up my database tables, and everything is as it should be.

I really don't recommend any user trying to do this, unless you an experienced developer.