[plt-scheme] version (200) of collection ("mzscheme") is required, but version (300) is installed

Hello,

I am confused by the error message produced by setup-plt when attempting to 
unpack the package that was packed by the previous version of PLT Scheme:

setup-plt: Setup version is 360
setup-plt: Main collection path is C:\Program Files\PLT_360\collects
setup-plt: Collection search path is
setup-plt:   C:\Documents and Settings\lizorkin\Application Data\PLT 
Scheme\360\collects
setup-plt:   C:\Program Files\PLT_360\collects
cannot install; version (200) of collection ("mzscheme") is required, but 
version (300) is installed

 === context ===
c:\program files\plt_360\collects\setup\unpack.ss:224:36
c:\program files\plt_360\collects\setup\unpack.ss:139:10

Why do I have to use the "--force" option for unpacking, even if I know the 
package to work for both PLT 2XX and PLT 3XX ?

Dmitry