The biggest differences are related to syncing performance, which is going to be significantly worse for you with address indexing enabled. There were also some database changes but that's not really a problem for you.
There were some database changes, which made this a pain.
I first downloaded the full blockchain using 0.15.1.
I looked more into the 0.15.1 documentation and found this:
Downgrading warning
The chainstate database for this release is not compatible with previous releases, so if you run 0.15 and then decide to switch back to any older version, you will need to run the old release with the -reindex-chainstate option to rebuild the chainstate data structures in the old format.
If your node has pruning enabled, this will entail re-downloading and processing the entire blockchain.
Old versions are old, they have known reliability and performance issues. 0.13.2 is vulnerable to DOS attacks (plus potentially other security issues, but I don't recall for sure), and it isn't getting updated for other changes so it will far further behind over time. I would recommend at a minimum that you setup two nodes-- one on current software, one running your special code-- and make the one with the custom code connect only to the current software. This way it's shielded from abuse that it might not be able to handle and it's easy for you to upgrade the external node.
As an aside, that address index patch that was floating around gave rare false results for me. I suspect that it could lose entries when there were reorgs, but I'm not sure if that was the cause or something else.
I was going to run two nodes and had setup the addrindex patched node to run on a VM. Due to some disk constraints (speed, capacity) I ended up deciding I would just run the patched addrindex node and use whitebind and whitelist in my bitcoin.conf so nobody but I can connect. You raised a point about vulnerabilities. Do you think the addrindex node is protected if I use whitebind and whitelist?
Also, what about the incorrect results you saw? What did you see and was it from this version: bitcoin-0.13.2-addrindex?
Thanks