yes, that's right !

- install "Magic ISO Maker"
- extract the navboot file AND(!) ... read below
- edit it with NavTweak
- drag&drop the edited navboot file into Magic ISO Maker
- "save as..." ISO
- burn the ISO file

The nav unit doesn't allow it to "update" with the same software as already installed.
So you have to fake the SoftwareVersion.
do the following: also extract the 2 .tbl files from the directoy:
<drive>:\V_2\RR\0101\BMWC01S
Open it in an HEX editor (like PSPad http://www.pspad.com/en/)

In the first line you find: 30 38 30 30 This means: 0800 So: Software-Version 08.00
Just edit the last 30 to 31 and you nav unit will think it's a new Software Version and you can update your V28 without prior downgrade to V27

The next time you update, you can edit it back to 0800 then 0801 again...