I'm contemplating a migration too, but I have objects in the image I
need to preserve/migrate as well.  Though you don't mention that as a
requirement, you may still find an in-place upgrade convenient.  Here
are a couple of tips:

You can update an image in place by filing in the update stream from
the server.  Find the squeak tab on the left of your image (or at
least, on the left of mine :) and hit "load code updates."  I think
that, depending on the history of the image, it will sometimes stop at
a certain version.  You might try "save as new version" off the main
menu if that happens, though I'm not really sure what the implications
of that operation are. (Usually when it hits a version boundary it
just asks you if you want to proceed to to the next version.)

Second, you can control the change set that changes go into, so
changes you file in are not mixed in with your own.  If you load
updates from the server, each goes into its own changeset.  If you're
filing in the changes yourself, see the "changes" item on the main
menu, and "create new change set" off its submenu.  A more visually
oriented alternative is to create a new project and switch to it, then
file in the changes.  Each project, by default, has a changeset with
the name of the project.