I think clients should consider two situations. One situation is where they have existing contracts and should look into loopholes under the Brexit and try to consider renegotiation. They should always try to find common ground with counterparts and look at the commercial impact of Brexit. if they have a new contract, they should always consider the worst case scenario, in order to be well prepared.
Clients should also look at tariffs and exchange rates, including customs procedures, considering who should bear what tariff exchange rate, with reference to EU laws and territory. They should also put in arbitration clauses, because they can then be sure that this will be enforceable in the future.
No one knows what will happen, but there will be changes because of Brexit. We can all hope for a soft Brexit, but be prepared for a hard Brexit without any rules. It is important to be prepared and keep in contact with counterparts in Europe or in the UK, starting negotiations around what will happen.