I have to say that its tricky but any upgrade works, regardless if it make snay changes or not.

If you see the content of V27 or V26 with Magic ISO , you can see that it has a folder for MKIII, another one for MKII, etc....

This disk is designed to work with any version.

The main problem is the NAV unit, which is very temperamental, I must say.

Mine in the last 4 days has crashed about 8 times, with GPS power going off. ( This happend when I swapped male & female languages).

After managing to eject cd (this takes a while & patience, see threat), I just inserted version 26 or 27 , and system was restored.

It took a while for the system to recognice cd...but it worked.