According to the Salesforce Revenue CloudImplementation Guide, any changes to the design document during the build phase should be communicated to the project manager, who will assess the impact of the changes on the project scope, timeline, and budget. The project manager will then decide whether to accept, reject, or defer the changes, and update the project plan accordingly. This is the best practice to ensure that the project is delivered on time, on budget, and with the expected quality and functionality. 

Implementing the lookup price rulesimmediately without consulting the project manager or the solution architect could result in errors, inconsistencies, or conflicts with theexisting design or other components of the solution. It could also cause delays or rework if the changes are not aligned with the customer’s expectations or requirements. Therefore, option B and option E are not appropriate.

Consulting with the solution architect first could help to expedite the updates to the design document, but it would still require the approval ofthe project manager and the customer before implementing the changes. Therefore, option D is not sufficient.

Communication to the customer that ongoing adjustments can be made as long as we are in the build phase could create confusion or unrealistic expectations about the project scope and timeline. It could also undermine the credibility and authority of the project manager and the solution architect, who are responsible for managing the project and ensuring the quality of the solution. Therefore, optionA is not advisable.