I completely understand that users using old versions will report old bugs and potentially chew up resources..... but....
I don't think that justifies forcing people to upgrade to a more current versions.
I was happily using: paint.net 4.0 (β 4.0.5168.12074), when suddenly my workflow was interrupted by my favorite image editing software forcing me to install software I was not ready to...
Letting a user know they are using an older version should be enough. Let them make their own timing decisions.
Anthony
