In terms of saving money, I think the best way is to keep the $0 line as it is until kena force upgraded, not ownself upgraded.
Of course, that is provided your Circles $0 plan number is not important and can be discarded or ported out in the event of kena force upgraded.
On the other hand, if the number is something that u give to others to call and sms, then might be good to ownself upgrade to the $5 plan now via the app since it offers 20gb data.
For me, I use my $0 plan number for its 1gb data only and if kena force upgraded I will simply terminate the plan. But I do have one number that I will want to port to another $0 plan, probably the M1 Maxx plan then.