What I did was to enable version control in Central Administration and on the Forms Server, but it was just the latter that I was supposed to do. To update the form I now upload a new document in the Form Templates folder. Uploading it through Central Administration was only necessary for the initial release.
Thank you for your help.
Edit: Well, seems I got another problem. When I update the form by uploading the published file on the Forms server, it stops being browser enabled, even though the uploaded file is identical to the one I uploaded in Central Administration initially. When trying to open the document in IE7 it wants to be opened in InfoPath, and if I try the same in Firefox I get the following error:
This
form template is not currently browser-enabled. It must either be
republished as a browser-enabled form, or opened using Microsoft Office
InfoPath 2007.