here is a solution you can try aslong as you have _ in your bbcable_ID field.
pos = InStr(1, bbcableid_result, "_")
st = Mid(bbcableid_result, pos + 1, Len(bbcableid_result))
NewBBCableID = "bbc_" & nextnr
Update the recode and everything else like you do except calculating newbbcableID.
HTH