I plan to upgrade my virtualbox to 4.3. But all my guest OSs (win7, linux) are created under 4.2.
Will the upgrade cause any problem in virtual machines? I do not want to reinstall all my guest machines.
You'll never have a problem when applying minor version upgrades to virtualbox. I only glance at the change logs for breaking changes that will usually be highlighted and flashing dayglow red. On major version upgrades, 3.x to 4.x for example, I will read the docs.
On the off chance the upgrade breaks something you just have to uninstall virtualbox, and reinstall the original version. Import the VMs. If you want to take precautions, you can copy your VM folder so you have a backup.