Hi Leslie,
Thanks for your reply.
I've downloaded the latest NitroPro13 version and tried to install it without un-installing my current registered version. Unfortunately the setup failed and has been terminated with an error. Firstly the download started but then it said: "Fehler beim Setup-Assistenten für Nitro Pro" (Error with setup assistant of Nitro Pro).
BTW, when originally installing Pro13 64bit, I used a msi file with about 207 MB in size, not a setup exe with about 2 MB. Further, should I firstly deactivate the licence followed by uninstalling the current version, before installing the new version? This would be not very user-friendly.