As time passes the system places less emphasis on the classification of a run, relying more on the recorded data only. Consequently, it is not necessary to trawl through your historical data to correct classifications.
Our guideline is to try to ensure that the last 1 to 2 weeks is accurate. Utilise Economy for easy runs and the Custom types for most other runs, e.g. Custom (Intervals) for interval sessions. For any runs that you are not sure of you can always mark as Freestyle.
The exception to the above 1 to 2 week rule are (best effort) races. Ideally these should be classified correctly as they provide benchmark runs of your capability given a block of training.