I'm not 100% sure I am right but I can learn if I answer, let's try then:
The question is, what do you mean by UTF-8 format string? Does it come from an UTF-8 encoded source and was decoded during import ("Text", "JSON" format etc.)? Or was it imported as a raw bytes from such source ("String", "Byte")?
Your question suggest the latter while the former is more likely to be the case. Anyway, for 'raw bytes' scenario you can use
FromCharacterCode[ToCharacterCode[string, "UTF8"], targetEncoding]
and for, a more likely, decoded string scenario:
ExportString[string, "String", CharacterEncoding -> targetEncoding]
I didn't have a coffee yet so sorry in case I made a mistake, strings and encoding can be confusing.