Any large project must begin with the collection of requirements, if the customer has not performed the same procedures before you. It is important to distinguish this stage as a separate one (before the TOR) to control the deadlines. Add the word "analytics" to the word combination "requirements gathering": it means that business requirements should be analyzed by the forces of the techieur (or someone else) to formalize them.
If there are high risks of changing the composition of the work (the customer does not know exactly what he wants), the analysis of the requirements, especially need to allocate a separate stage. Then you will have an opportunity for maneuver: new requirements at the analysis stage are added to the estimate, and then detailed TOR is already written on them.
It should be noted that there is often a temptation to sell the analytics as a separate service. In many cases, customers are not ready to buy it, because they do not understand the final result of the service. And here to make analytics within the limits of the project, then having corrected the estimate (reasonably and!) - it ok and does not cause questions.