You could modify your questionnaire when the survey is already in the field. Survey Solutions treats any modifications of a questionnaire as a new survey. Even if the changes are minor, the modified questionnaire will be shown as a new survey both on tablets and in the HQ/Supervisor application.
In general, we strongly advise against making changes in the questionnaire after the survey goes to the field. We do advise to plan extensive pre-testing and piloting survey prior to the field. You want to eliminate as many problems with the questionnaire and the survey design at the stage of piloting, so that when the survey goes to the field, no changes are necessary. Modifying questionnaires during the field work might result in incompatible data generated by the different versions of the same questionnaire. Even minor changes in the validation or enablement conditions might lead to disablement of the whole sections and an rerouting of the questionnaire flow.