As long as you are using QC you will be able to use the 'Asset Upgrade Tool For QC' (included with the QTP 10 installation files) to upgrade all your QTP 9.2 tests to QTP 10. If you are not using QC you will have to upgrade all the tests my hand (copy all code out and paste it into a new QTP 10 test)
The tests will still run as QTP 9.2 tests but you will not be able to edit them until you upgrade them to QTP 10 tests
- Once upgraded the scripts cannot be reverted back to old version. You should make sure you have a full backup of the script
- Make sure you run few of your script on QTP 10 before upgrading all of them
- You can upgrade all of them in one go by writing a AOM script which opens the scripts and then saves it
- If you have used Actions without relative paths then you might face issues with missing actions and stuff
2. I feel that no need to change Syntax but you need to search if you are using anything got deprecated by HP and used only for backward compatibility so you avoid to use them in future. You can find them in Help file.