Has there been any changes across core versions for the sendrawtransaction interface. I am trying to understand if upgrading the core version from say 12 to 15 needs a change in serialization. I'd also like to know if there are any versions across which such upgrade would fail.
sendrawtransaction takes transactions in the network serialization format. There is a new network serialization format for segwit transactions, so such transactions would be broadcast in one format for 0.12 and a different one in 0.15. If you are not making segwit transactions, then
sendrawtransaction has not changed and you can use it in 0.15 as you did with 0.12.