I have no clue if this would work or not, but if it were me I would try and upgrade to a lower version like 1.6.0.5 first, then upgrade that to 1.6.1.18. Or maybe even upgrade to 1.5.6.3, then to 1.6.0.5, then to 1.6.1.18. Try smaller upgrades, see if they work before moving on.