Thanks for the comments.
Given uncertain and conflicting information, I've decided the optimal strategy is:
1. Wait until 2degrees turn off the 2G network. The mobiles may or may not still work. Cost: $0 if the mobiles work.
2. If the mobiles don't work, then switch to Vodafone. Cost: $8 (4 SIMs).
3. If mobiles still don't work on Vodafone, then buy new mobiles. Cost: about $1000 for equivalent mobiles.