I dont know if the KC5 has the amss file required to restore prior functionality .
In any case ,you need a Stock European Froyo ROM ,specifically ,the
I5510XWKD4 firmware since it is the only FW with Froyo Bootloader .
You will need to flash KC5 firmware over the KD1 .
That is two flashes in total
Check the link below for Firmwares
SamMobile Firmware Page
KD1 is under Smartphone-->Android-->GTI5510-->Europe
KC5 is under Smartphone-->Android-->GTI5510M
Order of flashing
First Flash KD1 then KC1
just curious ,can you tell me which FW do you have right now . XEO-KPG had the least problems . All others are touch and go .