If your cutoff is every 10 the best date to recontract is 9, this is to avoid double charging if you have combo/booster in your account.
Globe's system seems to be working only by the cycle and by this I meant any changes you will make to your plan will take effect at the beginning of your next cycle or the day after your billing cycle. Unlike before their system upgrade, we could make changes to our plan anytime and get these changes activated in just within 24 hours or a few days after and their system will just adjust.
Even if your cut-off is every 10th of the month, you may process your recontracting anytime before that (as long as 30 days or less) and your new contract (and new plan if applicable) will still be reflected at the beginning of your next billing cycle. This same principle also goes for new SIM activation if you got a handset that requires a different cut of the card.
And as for MLP, promo packets are charged IN FULL regardless of when you are activated in your billing cycle.