+1/+1 counters aren’t “attached” to the creatures they’re on like an Aura or Equipment is, so a +1/+1 counter/Food token that becomes a creature stays on the creature its on. It still gives that creature a +1/+1bonus. It doesn’t give itself a +1/+1 bonus. Note that +1/+1 counters/Food tokens don’t have a mana cost, so their mana value is 0. Many ways of turning them into creatures will turn them into 0/0 creatures, so unless something else is raising their toughness, they’ll immediately spoil . . . er, die.