p4 edit -t +w //depot/mystream/UnrealEditor/... p4 submit -d "Changing type to writable."
This will add a new revision with the +w type; once people sync this down the files will be made writable. I recommend this first approach because it's the most straightforward.
p4 retype -t +w //depot/mystream/UnrealEditor/...
This will retroactively change all the existing revisions to +w. This requires admin permission, and the change won't be reflected automatically on sync (since as far as the clients are concerned they already have these revisions) until either a new revision is submitted or clients do something like sync to #none and then re-sync.