But if you look at the other gospels you can find parallel passages where it says "Kingdom of God" instead of "Kingdom of Heaven".
like these:
Mt 4:17 From that time Jesus began to preach, and to say, Repent: for the kingdom of heaven is at hand.
Mr 1:14 Now after that John was put in prison, Jesus came into Galilee, preaching the gospel of the kingdom of God,
Mr 1:15 And saying, The time is fulfilled, and the kingdom of God is at hand: repent ye, and believe the gospel.
Mt 19:14 But Jesus said, Suffer little children, and forbid them not, to come unto me: for of such is the kingdom of heaven.
Mr 10:14 But when Jesus saw it, he was much displeased, and said unto them, Suffer the little children to come unto me, and forbid them not: for of such is the kingdom of God.
Mt 18:3 And said, Verily I say unto you, Except ye be converted, and become as little children, ye shall not enter into the kingdom of heaven.
Mr 10:15 Verily I say unto you, Whosoever shall not receive the kingdom of God as a little child, he shall not enter therein.
Mt 19:23 Then said Jesus unto his disciples, Verily I say unto you, That a rich man shall hardly enter into the kingdom of heaven.
Mr 10:23 And Jesus looked round about, and saith unto his disciples, How hardly shall they that have riches enter into the kingdom of God!
(This one could be taken as being different)
Mt 5:3 Blessed are the poor in spirit: for theirs is the kingdom of heaven.
Lu 6:20 And he lifted up his eyes on his disciples, and said, Blessed be ye poor: for yours is the kingdom of God.
Mt 11:11 Verily I say unto you, Among them that are born of women there hath not risen a greater than John the Baptist: notwithstanding he that is least in the kingdom of heaven is greater than he.
Lu 7:28 For I say unto you, Among those that are born of women there is not a greater prophet than John the Baptist: but he that is least in the kingdom of God is greater than he.
Mt 13:11 He answered and said unto them, Because it is given unto you to know the mysteries of the kingdom of heaven, but to them it is not given
Lu 8:10 And he said, Unto you it is given to know the mysteries of the kingdom of God: but to others in parables; that seeing they might not see, and hearing they might not understand.
And there are more.
Based on this do you not think that they could be used interchangeably?