if you need to make ad hoc changes to the description of an activity - for example 'please don't post attachments in xx format' or something that may emerge after the activity has started - you cannot make changes to description without first clearing the activity completion.

Request is to allow certain changes to be made to the activity without first clearing the activity completion data.

(perhaps an 'additional description' field like when you make changes to an ebay item after someone has bid)