Will my data be save when I deinstall the previous version on the PPC?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 and should be deleted anually!
When you have used the mobile version together with the desktop version, you have also an "automated backup" on your PC as a result of the sync. From there you can migrate the data into the V6 format by using the migration wizard on your PC version.
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 use the inbuild backup function in the PC version