God made a choice before the foundation of the world who He would give grace to and have mercy upon (Eph 1:4) (Rom 9:15). These are what Jesus is refering to as the wheat which He planted. The field is the world (For God so loved the world... God loves the work which He is doing here upon the earth).
The tares are those who God has blinded and deafened(Isaiah 6:10 prophecy) or hardened(Rom 9:18) or left, given up to their own selves. These are those who Jesus is refering to as them who were planted by the devil.
The reason that He dis-allowed the early harvest is because either everyone He had chosen had not been born yet to be saved or there are still those alive that had not yet been saved and forgiven their sins which would result in uprooting of those He had planted to thier own distruction.
No where is the reference that wheat became tares or that tares ever became wheat. These are two different distinct people.
In the end at His appointed time, God will send anges out to first harvest the tares and cast them into hell and then harvest the wheat into His barns, up into heaven.
The Kingdom of God represents the world from Genesis to the end of Revelation. This parable was spoken in God's view of man. God knows who are His sheep therefore the distinct mention of wheat and tares. From mans view, we don't know who are wheat and who are tares therefore "whosoever believeth(from our point of view) shall not perish but have everlasting life.