The process for upgrading from MML v1.03 to WML v1.04 can have a number of 'gotchas' if you're not careful. Here's a list that I found as I did my upgrade...
1) Upload the new wmlv1_04.swf file to the directory that currently contains mmlv1_03.swf. If you need to give Flash SWF files special permissions on your server, make sure the new SWF file has those permissions.
2) Copy your mmlconfig.xml file to wmlconfig.xml. If you have multiple config files make sure you copy all the files.
3) Edit each wmlconfig.xml file and change the lines containing <mml> and </mml> to <wml> and </wml> respectively.
4) Edit your HTML/PHP/whatever you use to display MML and change all the references to mmlv1_03.swf to wmlv1_04.swf and references to your mmlconfig.xml file(s) to the equivalent wmlconfig.xml file(s). If you have multiple places you run MML from make sure you change all of them.
That was what I had to do. I have a fairly complex config for running MML so I had a lot of files to change, plus I had to research/write this note. My mods took about 45 minutes to make. I'd expect that most people will be able to make the necessary changes in about 10 minutes or less.
If you have problems, please post a report in the Bug Tracker at http://wxd.weather-watch.com/mantis/
so that we can make sure any upgrade issues are resolved ASAP.