What are the Chances of Uncontested Full Custody Being Switched to 50/50

My question involves a child custody case from the State of: Oklahoma

Divorce in '07
CP was awarded full legal/physical custody of children. NCP did not contest custody and occasionally exercised visitation rights.

NCP moved out of state (1000+ miles away) in fall '08 to pursue to other interests. NCP did not exercise visitation rights.

NCP pressed for long distance visitation rights in Spring '10 and occasionally exercised visitation rights over summer/holidays.

NCP plans to move back to Oklahoma with fiance in Spring '13 and file for 50/50 custody.

CP understands a modification can be filed. However, what does a judge review when deciding a change of custody status?

CP is cautious due to NCP history....waiting for other shoe to drop.

If NCP wants to change the current custody order, s/he will have to prove a significant change of circumstance.

So - other than the NCP moving back to the area, what's changed?

Based SOLELY upon what you've written here, the NCP has no chance.

What are some examples of a significant change of circumstance?

CP and children have stable homelife (CP's POV) and stable income (regardless of intermittent child support).
But NCP is master of spinning drama (past occurances before/during/after divorce) and is not beyond spinning tall tales.
CP is single (focus on children/job). Could NCP think having a fiance (two adults better than one idea) would give him a leg-up?
CP realizes fiance is driving force behind big plans.

Children are teens now. Would they play a part in court decision?

NCP needs a bit of a reality check.

NCP needs to show that something significant has changed in the lives of the CHILDREN.

In OK, their aging up is NOT a CoC in and of itself; so NCP will need that before s/he gets to court. If NCP can show a CoC (and nope, getting a new fiance doesn't matter in the slightest), then the wishes of teens will often carry at least some weight in a custody decision.

I'm wondering. Is NCP paying child support? I can't help (cynical, jaded, been at this for too long :D ) but wonder if the fiance isn't keen on paying child support...

NCP currently unemployed due to actual lay-off and state unemployment is providing partial CS (some better than none).

But, NCP has history of avoiding CS with voluntary unemployement/under-employment/under table/non-notification of employment/etc. since divorce. In arrears well over 10K and growing. Oklahoma CSE none-too pleased with NCP but backlogged. NCP did file for reduced CS in spring '12 but case is on waiting list.

CP is certain fiance is not keen on CS and stirs the pot.

CP still confused on what may qualify as a significant CoC.

As far as custody goes?

Child is failing at school under CP's care. And I mean going from an A to an E in one semester in several subjects.

Medical neglect.

Abuse.

CP is about to go to jail for a long period of time.

CP has just married or moved in with a convicted child sex offender.

CP is moving to a different country or state without permission (and this doesn't ALWAYS count).

CP gets several DUIs with the child present (which also comes under "abuse" or "neglect").

N/A on all of above.
Thanks Dogmatique :)