Version:           1.4.0-r2 (using KDE KDE 3.5.2)
Installed from:    Gentoo Packages
Compiler:          GCC 4.1 
OS:                Linux

After upgrading to Amarok 1.4.0-r2, my simple playlist files were converted to extended playlists while updating my library after the update.  This was not specified anywhere within the program, and appears to happen when "re-scanning" the collection and not when it automatically updates files that were recently added.

A sample from before and after (different albums but examples) are as follows:

BEFORE (without line numbers)
 1 01 - Funny In My Mind (I Believe I'm Fixin' To Die).mp3
 2 02 - Morning Dew.mp3
 3 03 - One More Cup Of Coffee.mp3
 4 04 - Last Time I Saw Her.mp3
 5 05 - Song To The Siren.mp3
 6 06 - Win My Train Fare Home.mp3
 7 07 - Darkness, Darkness.mp3
 8 08 - Red Dress.mp3
 9 09 - Hey Joe.mp3
10 10 - Skip's Song.mp3

AFTER (without line numbers)
 1 #EXTM3U
 2 #EXTINF:272,ZZ Top - Pincushion
 3 /mnt/array/ftp/Music/Downloads/ZZ Top - Antenna/01 - Pincushion.mp3
 4 #EXTINF:297,ZZ Top - Breakaway
 5 /mnt/array/ftp/Music/Downloads/ZZ Top - Antenna/02 - Breakaway.mp3
 6 #EXTINF:248,ZZ Top - World Of Swirl
 7 /mnt/array/ftp/Music/Downloads/ZZ Top - Antenna/03 - World Of Swirl.mp3
 8 #EXTINF:282,ZZ Top - Fuzzbox Voodoo
 9 /mnt/array/ftp/Music/Downloads/ZZ Top - Antenna/04 - Fuzzbox Voodoo.mp3
10 #EXTINF:250,ZZ Top - Girl In A T-Shirt
11 /mnt/array/ftp/Music/Downloads/ZZ Top - Antenna/05 - Girl In A T-Shirt.mp3
12 #EXTINF:283,ZZ Top - Antenna Head
13 /mnt/array/ftp/Music/Downloads/ZZ Top - Antenna/06 - Antenna Head.mp3
14 #EXTINF:237,ZZ Top - PCH
15 /mnt/array/ftp/Music/Downloads/ZZ Top - Antenna/07 - PCH.mp3
16 #EXTINF:278,ZZ Top - Cherry Red
17 /mnt/array/ftp/Music/Downloads/ZZ Top - Antenna/08 - Cherry Red.mp3
18 #EXTINF:349,ZZ Top - Cover Your Rig
19 /mnt/array/ftp/Music/Downloads/ZZ Top - Antenna/09 - Cover Your Rig.mp3
20 #EXTINF:309,ZZ Top - Lizard Life
21 /mnt/array/ftp/Music/Downloads/ZZ Top - Antenna/10 - Lizard Life.mp3
22 #EXTINF:246,ZZ Top - Deal Goin' Down
23 /mnt/array/ftp/Music/Downloads/ZZ Top - Antenna/11 - Deal Goin' Down.mp3

This bug was reproducible by re-scanning the collection, which is when it changes the playlist files.  The bug is further proven by the modification timestamp on the files as being roughly 11:49 AM while the upgrade to 1.4.0-r2 finished minutes before and the collection was immediately re-scanned.

The idea behavious would be that Amarok would not touch any playlists unless specified that the user would like them changed to another format.  The files and files that Amarok loads information from should ideally be treated as read-only, which was the case previous to this version.