If you avail the iphone 6 through retention (assuming it would be offered), then you can still retain your unlimited plan.
But if you are considering to get a new plan just for the sake of the iphone 6, then the parameters of the new GoSurf plan would apply.
Based on the iphone 5's Philippine release, the price of the iphones have been uniform with the telcos and local dealers. You might want to check the authorized local dealers which sell warrantied, unlocked units if they have attractive installment plans.
Sometimes, Globe will INSIST that you change your plan in order to get a new phone. It's up to you if you want to stand your ground or give in.