apt will only replace the current package of something if there is a newer package. Updates that come now will likely be to previous versions. It won't downgrade it unless they do something spectacularly silly with the versioning.
Lock version is not as clever as it sounds. It's supposed to do what it says on the tin, lock the version... But it only locks it within Synaptic. Anything else that does package upgrades (read: Update Manager, apt-get, aptitude, etc) ignores this. This is probably buggy behaviour so I would expect this to be fixed in time.
But to stop things getting upgraded, you want to "hold" it. To do that, load up aptitude, find the package (
/ is the quick-key for searching,
n finds the next instance) and press
h on it to hold it. This is respected a lot more than Synaptic's "lock" feature.
Edit: Another way of doing this via the command line is by running this:
echo <package-name> hold | dpkg --set-selections
But as I started with, I don't think you need to do this in your circumstances. Provided you're using a package version that is clearly newer than the current stable releases (and isn't just a random build number - you can see in Synaptic) you'll be fine.