Some Karmic applications will work in Lucid. It is hit and miss. Until a
couple of days ago I was using Getdeb repos which only had Karmic releases.
Then they switched to Lucid two days ago. Opera is still for 9.10 and I use
that repository. Some applications will not work because they use older
dependencies and can cause a conflict. Lucid's repos are the same as
Karmic's so an upgrade would upgrade all of the apps as well. Third party
applications may be a problem. You will have to upgrade them separately of
course and you are always taking a risk when you upgrade.