Higher land value means that your depreciation per year goes down but preserves your basis at disposition better -- yes ?
Since you are concerned that your depreciable basis is probably wrong ( if you use the county assessment ) AND because you have used the old depreciation only for only 2016 and 2017 ( i.e. within the statute of limitation ), there are two paths as I see it ( and without requiring IRS approval/ permission):
(a) proceed with as is -- no change. Generally IRS has little interest in verifying the depreciable basis. Unless you become randomly selected for a full audit for compliance. Very unlikely, unless you are very unlucky and even then you have a good logic why you continued plus IRS always loves ( or should love ) higher depreciation because of increased ordinary gain.
(b) change to the new split for 2018, amend the 2016 and 2017 returns ---- reason being land value change going back to 2016 by the county.
I think that either way is as good as another even though as a landlord looking at longer term effects, I would prefer to have less recapture amount when I ultimately sell the property or would like to use the property as main residence for two years before I sell so that I can exclude the gain ( even though the recapture may still haunt me ).
Good Luck
