There is generally no need to do incremental upgrades.
Actually you are even better off not doing any incremental upgrades. A few little issues in the upgrade code have been fixed over time, so the very latest upgrade code is the best. If you move to 64-bit, then just restore your 2.1.5 config, it will reboot and apply all the upgrade steps right up to 2.3.*
2.3.2 will be out very soon, so you might even wait until then, just to save you upgrading again soon after you get to 2.3.1_5