I have at least 2 versions of UltraEdit always installed. It is quite easy to isolate 1 version.
First make sure that all UltraEdit settings of UE v13 are stored in uedit32.ini and not in the registry - see Advanced - Configuration - Application Layout - Advanced - Use Registry for settings (not INI file).
For your usage it is required to enable for UE v13 Advanced - Configuration - Application Layout - Miscellaneous - Allow multiple instances. That is important because the macros should be always executed in a new instance of UltraEdit v13, independent if an instance of UE v15 currently exists.
On your computer with UE v13 create a new directory for v13, for example C:\Program Files\IDM Computer Solutions\UltraEdit13\.
Copy all files and subdirectories from C:\Program Files\IDM Computer Solutions\UltraEdit\ into this new directory. Copy also all files in %appdata%\IDMComp\UltraEdit\ into the same new directory. UltraEdit searches for uedit32.ini always in the program directory first before searching in %windir% and in %appdata%\IDMComp\UltraEdit\. To be more precise: If you start C:\Program Files\IDM Computer Solutions\UltraEdit13\uedit32.exe, UltraEdit first evaluates the command line parameters for /i="...", then the environment variable UEINIR, then it searches for C:\Program Files\IDM Computer Solutions\UltraEdit13\uedit32.ini. If you would rename uedit32.exe to ueditv13.exe, UltraEdit would search for ueditv13.ini in the same directory as the EXE. So UE v13 uedit32.exe uses now always uedit32.ini in the program directory of v13.
UE v13 is now isolated. You have to use the new full path to uedit32.exe in the new program directory of UE V13 in your batch files. You can now install UE v15 over the existing v13 in the standard program directory and continue to use the settings of UE v13 which are updated on first run to v15.00. After installing UE v15 you should uncheck the Allow multiple instances - stored in %appdata%\IDMComp\UltraEdit\uedit32.ini used now only by UE v15, if you don't want that for UE v15.
If you work with UEINIDIR by default, you would need to change the value of this variable in your batch file or use additionally command line parameter /i="C:\Program Files\IDM Computer Solutions\UltraEdit13\uedit32.ini". It is no problem to change the value of UEINIDIR during execution of a batch file because a batch file always runs in its own environment and therefore the environment variable UEINIDIR of the Windows system is not changed when you change the local UEINIDIR of the command environment of the batch file. If you normally don't work with environment variable UEINIDIR, you can ignore everything in this paragraph.
You might need to move the file with the registration data of UE v15 also from %appdata%\IDMComp\UltraEdit\ into the program directory of UE v15. The other configuration files can remain in the users application data directory.
Registering: Buy the upgrade license. UltraEdit licenses are per user licenses. You have paid for v13, you pay for v15. So you can use both at the same time, as long as you are the only user which uses UltraEdit v13 and v15. That you have used a special discount for v15 does not matter. This discount is not granted only for users which uninstall the existing version. It is granted for users already having paid a license for a previous version, independent if the old version is currently installed or not, or remains after installing the new version and is further used by you.