Hi,
Yes it is possible.
The upgrade would work, it would be like a classical update but, some files of the business version would stay. It wouldn't be a problem because the starter version would not use them.
Your overrides would stay, you database elements would stay too.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.