Upgrading an altcoin (LTC fork) from 0.8 to 0.15

Hi folks,
I'm upgrading an altcoin which originated as an LTC fork. The current release is based on LTC 0.8.
I'm wanting to upgrade this to 0.15.

I’ve got 90% running - wallet loads, syncs blocks from the right network...even receives transactions.

My issue though is that I’m unable to send transactions. The network rejects my transaction and leaves it stuck in my wallet’s mempool. No other node gets it. If I try to send the raw transaction from an older 0.8 based client on another machine (known working environment), it kicks back with error 64.

I made sure to keep bip66 and segwit activation for sometime in the future, and mostly just want to update to the latest release for compatibility reasons. BIP66 and segwit will be planned upgrades.

Any thoughts on why my sends are failing?