… YouTube will accept it only as a new movie, not as a replacement for an existing movie.…
this is correct (and should be 'fixed' by YT!).
another option is:
YT itself offers some very rural edit options with uploaded movies. If you have for instance just to chop-off a piece, this can be done straight on YT.
When I've made the changes to the text and then press play, it still shows the old text.
There is a bug in iMovie '11 (and earlier versions) where changes to titles are not always saved. I've had instances where, after changing text on a title, the revised title plays back fine in the viewer. But, in the shared (exported) version the original title plays. There have been many complaints about this on the forum but no real fixes have been forthcoming.
Even after clicking on "Done" after amending a title, the changes are not always saved (although they appear to have been saved prior to exporting). My method is to delete the title, then apply it again with the preferred style and text.
Regarding publishing to YouTube -
I thought I'd try and 're-publish to YouTube' which is an option under the YouTube logo at the top of the project, but I get an error (-50).
A better option will be to export the movie from iMovie, then upload the exported file to YouTube using YouTube's built-in uploader.
In iMovie select Share > Export Movie and choose an appropriate size/resolution, such as Medium, Large, HD 720p or HD 1080p, depending on what size your project supports (any unsupported sizes will be greyed out). Give it a name and save location (say, Desktop).
The exported file will be in H.264 format with the extension .m4v or .mov depending on the size exported. The H.264 format is ideal for YouTube (in fact, it is the same format as generated by iMovie's YouTube export option).
In your YouTube account, use the built-in uploader to navigate to your exported movie and upload it. This method seems to be more reliable than using iMovie's uploader.