Will my data be save when I deinstall the previous version on the PPC? (V5)

Yes! The deinstallation of your PPC will just delete the application files that were installed initially. All data files will stay on your mobile device! When you have used the mobile version together with the desktop version, you have also an "automated backup" as a result of the sync. That means even in a case that you will start from scratch with a blank PPC system and have already data on your desktop, SiDiary will sync it back to your mobile device!

Generally it isn't a bad idea to backup your data frequently especially when you want to update your system. In case of SiDiary you just need to copy the files from the data-subfolder (either PPC or PC) to a backup folder, that's all.