Is it safe to add a new Mongo 3.2 node to a replica set that is running 2 nodes + arbiter on version 2.6?

[Davis Ford]

I would like to upgrade the other nodes, but I cannot take the system down.  The idea is to add a new (updated) secondary, then take the old secondary offline for upgrade, then swap the primary over, and upgrade it.

Anyone see any gotchas with this?  We are on the old storage engine, and I planned on moving to the WiredTiger with the new versions.  Will the replica set / oplog process handle that or will there be some incompatibility?

Thanks in advance,

Davis

[Davis Ford]

Welp, the answer to this was a big resounding "NO".  I totally missed this when researching the topic.  https://docs.mongodb.com/manual/release-notes/3.2-upgrade/

> To upgrade from a 2.6-series release, you must upgrade to the latest 3.0-series release before upgrading to 3.2. For the procedure to upgrade from the 2.6-series to a 3.0-series release, see Upgrade MongoDB to 3.0.

I built the 3.2 and added it, and it generated a whole host of errors in the logs.  It did not appear to do any damage other than have me waste a whole day of my time.  Now I have to go back and build it for 3.0 and then migrate all my nodes to 3.0, then migrate them again to 3.2.

I wish I had known this, and had not missed that notice on the upgrade path.  I'm posting here hoping it might save someone else the headache I endured.