MWL#55 :

Allow MSI and  NSIS side-by-side installation if  installed NSIS package
differs in  "major.minor" version numbers. Still disallow MSI and NSIS 
if major.minor versions  of both packages match.

win/packaging/extra.wxs.in

@@ -637,7 +637,7 @@
       Message=
       'Previous version of MariaDB was found, that used incompatible installer.
Please remove "[NSISINSTALLKEY]" before you proceed with this installation.'
       >
-      NOT NSISINSTALLKEY OR Installed
+      <![CDATA[ NOT(NSISINSTALLKEY << "MariaDB @MAJOR_VERSION@.@MINOR_VERSION@.") OR Installed]]>
     </Condition>
     <Condition Message=
        'Setting the ALLUSERS property is not allowed because [ProductName] is a per-machine application. Setup will now exit.'>