Filters get messed up as Issue update time gets changed when workflow gets updated

We had got 10 jira projects associated with a workflowscheme.This workflow scheme was associated with these projects when we had set up jira in our organization in November,2005..After one year, we required a change in our workflow.We changed the workflow and associated the new workflow scheme with those 10 projects.But this created havoc in our organization at management level who used to review the progress of the issues created for these projects.They had created the filters based on the updated time of the issue.Now around 18000 issues have the updated time as the workflow update time!!!!
We did not expect that JIRA admin changes (like workflow modification) would affect 'Last updated' field of all Issues related to the concerned project.We believe that JIRA Admin Update must be decoupled from the Issue Update.