You will clearly want to ensure that any position that relies on EU law crystallises before Brexit.
Your question, however, raises a wider issue. After Brexit, courts will still be ruling on the pre-Brexit legal position for some time to come, and there could be significant differences in how a post-Brexit court applies pre-Brexit law.
For example, it is unlikely that a post-Brexit court would have recourse to the CJEU. This is significant because it is far from clear that the UK courts would always reach the same view as the CJEU. The UK’s position on CFC rules, transfer pricing and corporate tax groups might well have found a more receptive audience in the UK courts than it did with the CJEU. Until prompted by rulings from the CJEU, there has been little appetite from within the UK to look at rules on VAT grouping. Even the approach of the CJEU, applying principles such as the Halifax abuse doctrine with regard to VAT, has diverged from more traditional UK rules of statutory interpretation.
As EU law becomes of increasingly historical interest, a post-Brexit court may well take a different view of pre-Brexit law than would be adopted by the CJEU.
This is also an issue if you are involved in existing litigation. In Littlewoods, for example, the question of restitution involving matters of EU law is still to be ruled on by the Supreme Court. The closer we come to Brexit, the less that courts may concern themselves with what the CJEU might say.
In fact, although we might assume there would be no retrospective change to today’s laws, the UK has not historically been shy to impose rules that have amended limitation periods and capped accrued tax claims. The CJEU has generally rejected such provisions. Post-Brexit, the UK may feel more securing in pursuing them.
All this means that, if you are looking at rules informed by EU law, you should consider how the UK courts (unburdened by CJEU oversight) would view them. You may also want to explore ways of resolving any uncertainty or dispute before Brexit takes hold, whether through formal proceedings or routes such as ADR.