Is there a way to force Assets to redo transforms?
I can't actually reproduce your problem but here's a couple of things to check and try:

Clear your browsers cache!
Did you re-upload through Assets with the same filename and click replace?
Purge all the caches via the Clear Caches button on the settings page
Update Asset Indexes on that same page
Failing those, manually delete the contents of the /craft/storage/runtime/assets and /craft/storage/runtime/cache folders
You can also try deleting the generated images manually in web/assets/..
If you're still weeping into your coffee try emptying the craft_assettransformindex table in the database

Of course, back everything up before you try all this...