In my opinion, there are three types of version changes for Joomla:
Joomla 1.5 to 2.5 or 3
This could be called a migration, because a new installation of Joomla is required, and all content, users, extensions etc. has to be migrated or moved to the new site.
Joomla 2.5 to 3
Going from one major version to another (i.e. 2.5 to 3.x) is usually called an ...
Bit of a tricky one this is. Too many words to describe 2 different thing.
Personally I think we should stick with one of the following options:
To make life extremely simply, keep 1 tag (upgrade) to define everything. This can be used for question regarding upgrading in general, for example:
Joomla 1.5 to 2.5 or 3.x
Joomla 3.2 to 3.3.x
For the second ...