Nothing will happen after this date and Aspose.Slides will continue work without watermark.
Watermark will return if you upgrade to version released after 2006-09-20.
You can check these wiki pages to get more information about licenses and subscriptions:
Just to verify...I changed the "widget" from .Powerpont to .Slides without getting a new license, just using the existing one we had with .Powerpoint.
So if I understand you correctly, that is fine, but if we upgrade the .Slides "widget" again from your website, after 2006-09-20 (when our license expires), then the watermark will return.
And if we leave it alone from this point on, it won't...correct ?