Arjan - If you use version 2.8 Field Organiser to re-type a field/column from Text to the more restrictive, but useful Category, then there should not be any change in DataExplorer views where the data typing for the field was previously in 2.7 changed from Text to Category on Load to Omniscope (inside blue box) using Data > Manage fields

This option was added to version 2.8 to deal with the following use case:

http://forums.visokio.com/discussion/2157

You can decide if there are any advantages to changing previous reports and/or test your version 2.7 reports by adding a new Field Organiser just prior to Load (the blue box labelled Omniscope), and re-typing the fields to be tested from Text to Category...you should not need to remove the existing 2.7 Text to Category re-typing operations, they should have no effect, but you should be able to remove them to speed things up a little.

If you use this feature on a new file, there should be no problems declaring text values with relatively low numbers of unique values...up to say 200, as Category...assuming subsequent Appends do not increase the number of unique values in the Category field to a number that would degrade visualisation and filtering performance.