What postings occur when you alter the estimated value of a vehicle specification line?

 
Actual cost is higher than estimated cost
If the actual cost comes through higher than the estimated cost the system will use actual cost value to generate postings when it is invoiced (some lines may post earlier eg at A status however principle remains the same).  When the vehicle is invoiced the profit is correct on the vehicle as it has used the higher actual cost.  Changing the estimate cost to equal the higher actual cost generates no postings however by making the estimate equal to the actual the vehicle specline will drop off the KUO report.
 
Estimated cost is higher than the actual cost
If the actual cost comes through and it is lower than the estimate cost at the time of invoicing the system uses the higher estimated cost to generate postings (assuming there may be additional costs to come through).  If the estimated cost is inflated and no further costs are coming through then you reduce the estimated cost to equal the actual cost to correct the profit.  At that time the system will post the difference between the estimated and actual cost. Eg Est cost = $120, Actual cost = $100, reducing the estimated cost to $100 will result in a $20 Cr to COS (or whichever account that particular spec line posts to).
If you do not manually reduce the estimated costs before the vehicle closes (moves to X stats) then at the time a vehicle does move to X status if there are any estimates higher than the actuals the system will post the adjustment entry to correct the overinflated costs that were used to generate postings at the time of invoicing.  It is better however to manually do this before the vehicle moves to X as this could be a few months down the track and ideally you want to reflect the true profit as soon as possible.
Best practice the KUO Estimates vs Actuals report should be run weekly.