Yes, existing customers have a 12-month grace period before they're impacted by a price change... but that clock just started ticking. There isn't an indefinite opt-out for this model change.
> Will GitHub force me to move to per-user pricing after 12 months?
> No. At this time we are not enforcing a timeline to move and if in the future we do decide to set a timeline we are committing to giving you at least 12 months.
[0] https://github.com/blog/2164-introducing-unlimited-private-r...
"Will GitHub force me to move to per-user pricing after 12 months?
No. At this time we are not enforcing a timeline to move and if in the future we do decide to set a timeline we are committing to giving you at least 12 months."
So you have an indefinite period of time + 12 months, not a hard 12 months starting now.
No you don't have to leave GitHub now. GitHub isn't forcing existing customers onto the new pricing, and it says in the post that if that changes at least 12 months notice will be given.