You can definitely change your remaining consumable to Combos, provided it's still enough. All you have to do is call the hotline, (for me, atleast 3 days before the cut-off date) to request for availment of combo's.
However, registering to Boosters, are charged on top of your monthly plan.
If you text GoSURF299 to 8888, it is a booster availment, not a combo availment, and success in registration gets you charged with an additional 299 in your monthly bill.
Hi sigbadass yes you can use your remaining PVs to avail combos as long as enough to get one. Just call the hotline of your plan for combo changes, don't text to any postpaid promos since this will be automatically charged on top of your monthly recurring fee.