From a legal point of view, the UK ceased to be a member of the EU from 1 February 2020. Entry into the Transition Period means that little has changed in practice since that date, but the end of the Transition Period on 1 January 2021 will see real changes in cross border trade that will affect commercial contracts that have an EU dimension.
Territory: if you have not already done so, you should review your key commercial contracts to check any references to the EU as a geographic territory (common in distribution agreements for example) or references to EU laws and standards which will no longer apply after 1 January 2021 and assess what effect this may have and where any changes will need to be agreed with your counterparty.
Regulatory: consider in particular the effect of a divergence (although likely gradual) by the UK from EU regulatory standards, particularly in relation to heavily regulated goods such as foods and medicines but also in areas of product safety, rules around consumer rights.
Staffing: changes to rules allowing free-movement of people between the UK and the EU may have a substantial effect in different areas of a contract. Consider any provisions relating to key staff for example, or resourcing issues more generally.
Financials: the knock-on effect of changes to trade rules for goods and services mean a potential for the economics of a contract to change. Increased costs will be likely in some degree as a result of predicted longer border-crossings, changes to customs and VAT regimes.