Did Jesus preach One God and follow the commandments to the Jews, and another message to the Romans? When reading the bible you read Jesus telling people to follow the commandments, whoever breaks even the least of the commandments will be in the lowest level in the kingdom.

http://www.biblestudytools.com/matthew/5-19-compare.html


  "Whosoever therefore shall break one of these least commandments, and
  shall teach men so, shall be called least in the kingdom of heaven:
  but whosoever shall do and teach them, he shall be called great in the
  kingdom of heaven."


He is preaching One God, and to follow the commandments.

Did his message fundamentally change after his death to the Romans?

How to you reconcile these 2 very different messages?
I believe that you may be misreading the scripture.
These Scriptures elaborate on the subject:

Jas 1:25  KJV But whoso looketh into the perfect law of liberty, and continueth therein, he being not a forgetful hearer, but a doer of the work, this man shall be blessed in his deed.
Jas 2:10  KJV For whosoever shall keep the whole law, and yet offend in one point, he is guilty of all.

you may compare that to saying she was only a little bit pregnant. The meaning here being that you cannot be only slightly guilty of breaking the Law, any more than you not really a sinner because all you did was to tell a white lie.
I have taken the liberty of quoting a couple of the more prominent commentaries here
David Gusik's commentary:

Whoever therefore breaks one of the least of these commandments: The
commandments    are to be obeyed as explained and fulfilled by Jesus'
life and teaching, not as in the legalistic thinking of the religious
authorities of Jesus' day. For example, sacrifice is commanded by the
law, but it was fulfilled in Jesus, so we do not run the danger of
being called least in the kingdom of heaven by not observing animal
sacrifice as detailed in the Law of Moses.
Whoever does and teaches them, he shall be called great in the kingdom
of heaven: The Christian is done with the law as a means of gaining a
righteous standing before God. One passage that explains this is Gal.
2:21: For if righteousness comes through the law, then Christ died in
vain. However, the law stands as the perfect expression of God's
ethical character and requirements.
The law sends us to Jesus to be justified, because it shows us our
inability to please God in ourselves. But after we come to Jesus, He
sends us back to the law to learn the heart of God for our conduct and
sanctification.

John Gill's commentary:

Whosoever therefore shall break one of these least commandments,....
Which are to be understood not of the beatitudes in the preceding
verses, for these were not delivered by Christ under the form of
commandments; nor of any of the peculiar commands of Christ under the
Gospel dispensation; but of the precepts of the law, of which some
were comparatively lesser than others; and might be said to be broke,
loosed, or dissolved, as the word here used signifies, when men acted
contrary to them.
And shall teach men so; not only teach them by their example to break
the commandments, but by express orders: for however gross and absurd
this may seem to be, that there should be any such teachers, and they
should have any hearers, yet such there were among the Jews; and our
Lord here manifestly strikes at them: for notwithstanding the great
and excellent things they say of the law, yet they tell us, that the
doctors of the sanhedrim had power to root anything out of the law; to
loose or make void any of its commands, for a time, excepting in the
case of idolatry; and so might any true prophet, or wise man; which
they pretend is sometimes necessary for the glory of God, and the good
of men; and they are to be heard and obeyed, when they say, transgress
anyone of all the commands which are in the law {h}. Maimonides says
{i}, that the sanhedrim had power, when it was convenient, for the
time present, to make void an affirmative command, and to transgress a
negative one, in order to return many to their religion; or to deliver
many of the Israelites from stumbling at other things, they may do
whatsoever the present time makes necessary: for so, adds he, the
former wise men say, a man may profane one sabbath, in order to keep
many sabbaths. And elsewhere {k} he affirms,
&quot;if a prophet, whom we know to be a prophet, should order us לעבור על
אחת מכל מצות, &quot;to transgress anyone of the commands&quot;, which are
mentioned in the law, or many commands, whether light or heavy, for a
time, we are ordered to hearken to him; and so we learn from the
former wise men, by tradition, that in everything a prophet shall say
to thee עבור על דברי תורה, &quot;transgress the words of the law&quot;, as Elias
on Mount Carmel, hear him, except in the case of idolatry.''
And another of their writers says {l},
&quot;it is lawful sometimes to make void the law, and to do that which
appears to be forbidden.''
Nay, they even {m} say, that if a Gentile should bid an Israelite transgress anyone of the commands mentioned in the law, excepting
idolatry, adultery, and murder, he may transgress with impunity,
provided it is done privately. You see what reason Christ had to
express himself in the manner he does, and that with resentment,
saying,
he shall be called, or be the least in the kingdom of heaven; meaning
either the church of God, where he shall have neither a name, nor
place; he shall not be in the least esteemed, but shall be cast out as
a worthless man; or the ultimate state of happiness and glory, in the
other world, where he shall not enter, as is said in the next verse;
but, on the other hand,
whosoever shall do and teach; whose doctrine and conversation,
principles and practices agree together; who both teach obedience to
the law, and perform it themselves: where again he glances at the
masters in Israel, and tacitly reproves them who said, but did not;
taught the people what they themselves did not practise; and so were
unworthy of the honour, which he that both teaches and does shall
have: for
the same shall be called great in the kingdom of heaven; he shall be
highly esteemed of in the church here, and be honoured hereafter in
the world to come. The Jews have a saying somewhat like this;
&quot;he that lessens himself for the words of the law in this world, נעשה
גדול, &quot;he shall become great&quot; in the world to come {n},''
or days of the Messiah.

Hope this helps.