My MPx200 originally came with SP2002 installed - I haven't tried to flash the ROM to anything other than SP2003, and certainly not from WM5 to SP2002, so I'm not sure I'm much help. Your best bet would be on the Modaco forums where there is an entire
forum devoted to the MPx200!
For what it's worth, my experience with flashing from SP2002 to SP2003 was a relatively painless experience, although I did worry about bricking the phone!
Just watch out for which ROM inage you have because there are non-English ones out there. Also, the process itself takes longer than you expect
(40 minutes?
) so you need to have a fully charged battery before you do it. When you reboot for the first time, be very patient because it takes a very long time
(five minutes?
) to reinitalise and get to the desktop.
The SP2003 ROM is great for me because I feel it runs more stably than SP2002, and there is also much more software available. You do sacrifice the ability to send MMSes, however, although there is a workaround for this
(which I have not used
), and I believe there are other quirks as well. SP2002 may be a better bet depending on how you use your phone.