In Webtrekk Q3, by default a change of a custom metric/formula does not affect Q3 reports that contains this metric/formula. The reports use the metric as it was defined before. The new metric definition applies to analyses and reports created after the metric change. Changes will only affect existing reports with this metric/formula if the checkbox at "When checked, made changes will also be applied everywhere in Q3 for this account (e.g. in reports/bookmarks)" is checked.
In Webtrekk Analytics a metric/formula change will always affect existing reports with this metric/formula. See note "Any changes made will always take immediate effect in the Suite for this account (e.g. in reports/bookmarks)" above the mentioned checkbox.