10.2 seems to have changed the $PATH completely, not sure why. So if your applications are installed in /usr/local/bin you will not be able to run them like you did before. Just update your path to include /usr/local/bin and everything will go back to normal.
As far as MySQL is concerned, it will stop functioning when you install 10.2 [because the /usr/local/bin isn't in your $PATH any more] but its easy to make it work again. I have been using Mark Liyanage's MySQL installer to simplify things. I got my old version of MySQL running [by just running the mysqld manually, logging and then running a mysqldump, instructions here:
http://www.entropy.ch/software/macosx/mysql/remove-old-mysql.html ]
This creates a text file containing SQL code to reload all of your data when you get MySQL upgraded. After I had the sql dump, I backed it up and then ran Marks uninstall script which removes every single mysql file that was installed with his previous package.
Next I installed the new mysql, logged in, dropped the mysql table [MySQL willcomplain when trying to load the mysql dump file because the mysql table already exists], loaded the mysql dump and then flushed privilidges.
Everything was back just the way I left it [I run a forum on my mac]. MySQL was upgraded as was OSX.
Feel free to IM me if you have any questions about this.