How to install / run with existing bitcoind full node already in place about documentation HOT 1 CLOSED

There is, it's at the bottom (Reindex section) of https://github.com/CounterpartyXCP/Documentation/blob/master/Installation/bitcoin_core.md (which is "mirrored" somewhere on the project web site as well, I think).

As the page explains, you need to use the same or newer version of Bitcoin Core addrindex and let it reindex the blockchain (meaning, if you are already on v0.13, I'm not sure Bitcoin Core addrindex v0.12.1 (currently the latest version) will be able to read the block if the format has changed (please check Bitcoin Core docs; I think sometimes the blockchain format is upgraded and can't be read by a lower version, other times it remains the same; if not, you'd have to download a new copy of the blockchain or wait for Bitcoin Core addrindex 0.13.1 (likely to appear 4-8 weeks after the official 0.13.1).)

If you have 0.13 and need to re-download for 0.12, you can setup Bitcoin Core addrindex 0.12.1 to connect to your existing instance (addnode=...) - that way download will be faster, but you need to have 2x space to do this. Also in this case you wouldn't reindex after download, since you can enable addrindex and txindex in the new instance from the very beginning.

from documentation.