You can configure the IDM Search Screen to automatically perform the search (based on the search filter criteria) and furthermore you can configure it to automatically pull up the object details in case of only a single result. This means if you perform a search on an object (let's say a User) and only one result is returned then IDM will not present the search result table page with only that one result but in fact will bring up that User's details page immediately.
To do that you can modify your search screen def to:
Automatically perform search - ON
Automatically set subject of task when there is only a single search result - ON
Save search filter - OFF
In case of only a single returned result then you will have no way to ever change the filter. If this is applied to a task (say a Modify User Task) then every time you perform that task the auto-search will come in, find one result (since filter is predefined), then immediately bring up that object (since setSubject is 'on') and so you will always result in that same one object as a result of the search.
There is and won't be a way for you to change the filter so that you can retrieve other objects and modify them.
The only way to "fix" this would be to modify the Search Screen definitions again, change the auto search and the auto filter and apply. This will get the search back out of the single result situation and allow the ones using the task (say the Modify User Task) to actually add a filter and perform a search.