To directly answer your question...
The Lisbon Treaty modified the way in which the EU treaties can be amended, with Art. 48 of the Treaty on European Union (TEU) creating a new system for treaty reform at EU level prior to an eventual intergovernmental conference and national ratification. (source)
Ordinary revision procedure
- The Government of any Member State, the European Parliament or the Commission may submit to the Council proposals for the amendment of
the Treaties. These proposals may, inter alia, serve either to
increase or to reduce the competences conferred on the Union in the
Treaties. These proposals shall be submitted to the European Council
by the Council and the national Parliaments shall be notified.
Simplified revision procedures
- The Government of any Member State, the European Parliament or the Commission may submit to the European Council proposals for revising
all or part of the provisions of Part Three of the Treaty on the
Functioning of the European Union relating to the internal policies
and action of the Union.
So yes, it would be legally possible to amend any of the treaties although the process would be difficult and likely slow. And No (see the rest of my answer), the EU could not make it more difficult for the UK to Brexit (this is an unilateral legal and valid decision from a member).
A comment about revoking Article 50...
Article 50 does not have a "revoke" clause, meaning that the UK cannot unilaterally revoke Brexit. There is no article or clause to be invoked to achieve this. This would either be done with consent from the other members, or at most be sent to the European Court of Justice (CVRIA; the latter being exactly what happened, check the edits at the end of this answer).
It can, however, unilaterally decide to leave the EU as is stated in the very first point of the Article 50:
- Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements.
That being said there is nothing the EU can do to avoid Brexit other than presenting itself and its advantages as a better alternative. If the UK wants to leave the EU all it has to do is what it has already done (invoke Article 50), and two years later is out of the EU.
If the UK wants to leave EU but still have access to some of its features (single market for example) it will have to enter negotiations. And this is precisely the part where the EU can take a stronger or softer stance. Currently the EU seems whiling to offer what has been offered to other nations (such as Norway if free movement of people is acceptable for the UK, or as Canada if not).
EDIT: I think the analysis made by David Allen Green is also relevant. Check here.
EDIT 2 (24, Sep., 2018): There is some new information regarding the potential revoking of Article 50 (EUobserver):
Scottish judges have asked the EU court to rule whether the so-called
Article 50 process, on the UK leaving the EU, can be "unilaterally
revoked" if the British parliament voted to reject Britain's exit
The same has been reported in other media-outlet such as BBC and Reuters. So this question should have a definite answer from the EU court in the next few months.
EDIT 3 (28, November, 2018): Regarding the revoke of article 50: The EU Council's most senior lawyer, Hubert Legal, told a hearing at the EU court in Luxembourg the UK could not unilaterally revoke Article 50. However it's important to notice that this is still awaiting an official decision by the EU court of Justice. (EUobserver)
The EU Council's most senior lawyer, Hubert Legal, told a hearing at
the EU court in Luxembourg the UK could not unilaterally revoke
Article 50 - the EU treaty exit clause that it triggered after the
2016 referendum. "National processes cannot suffice to pull the carpet
on which everyone has been forced to stand on," he said in a case
brought by anti-Brexit campaigners. A verdict is unlikely before 2019.
EDIT 4 (4, December, 2018): The EU Court of Justice advocate general just shared his opinion on the subject of revoking Article 50: The U.K. has the power to unilaterally revoke the notification of its intention to leave the EU before it quits on March 29, 2019, the European Court of Justice’s advocate general said Tuesday. (Politico.eu):
The top lawyer rejected the argument put forward by the European
Commission and Council that Article 50 only allows a country to revoke
its notification in the event of a unanimous decision of the European
The opinion is not binding but gives an indication of how the EU’s top
court may decide the case, which was brought forward at the request of
members of Scottish parliament, MPs and MEPs and several courts.
As mentioned by James K there's more to the advocate general recommendation to the Court of Justice. Here is the comment from the CURIA Press Release about this subject:
In answer to the question from the Scottish court, the Advocate
General proposes that the Court of Justice should, in its future
judgment, declare that Article 50 TEU allows the unilateral revocation
of the notification of the intention to withdraw from the EU, until
such time as the withdrawal agreement is formally concluded, provided
that the revocation has been decided upon in accordance with the
Member State’s constitutional requirements, is formally notified to
the European Council and does not involve an abusive practice.
EDIT 5 (10, December, 2018): I guess it's official. The ECJ just ruled that the UK can unilaterally revoke Article 50. (The Guardian)
The European court of justice has ruled the UK can unilaterally stop the Brexit process, in a decision that will boost demands for a
second EU referendum.
The court concluded that any EU member state can revoke the article 50 process without needing approval from every other member
state, in an emergency judgment timed to coincide with Tuesday’s
critical Commons vote on Theresa May’s Brexit deal.