It is all relative, in my opinion. A new version of a theme doesn't always mean you have to upgrade. If the features are that significant, and you think they will benefit your readers, then you will have to invest some time in porting the changes over to the new version. There is no "one-click" theme upgrade. If there was, that would be a feature of the theme, not WordPress. Many of the newer styled themes employ an options page that allows for customization without editing code, and would possibly allow for this type of upgrading.
The easiest way I've found to do such is to download the new theme, and then copy in the changes you made to the old one. Download a copy of the old theme for backup, delete it off the server, then upload the new version. You can then always make any additional changes needed, and if the new version just doesn't suit your fancy, you can always delete the new version, upload the old, and go about blogging.
Out of curiosity, which theme?