When we talk about how well websites perform post upgrade in the search engines, we are primarily talking about Google as the search engine. Google is of course the biggest search platform and accounts for around 80% of the UK market.
From the many sites that we have already upgraded, we have noticed almost no immediate exact match change from moving their sites to .uk . For example the search results show no difference in positioning of the site RoyalMarines.uk for the term "Royal Marines". We have noticed a slight increase in the amount of traffic coming in through long tail searches.
We do have more exact and up to date data that we sell to corporate companies who need more information before deciding to switch.
In general terms what you should expect is to see no drop in search result rankings but a slight increase in the number of people who click on the .uk brand as it stands out a little more in the ocean of .co.uk's that appear on that page.
This is assuming that the upgrade is done correctly and done right first time. Failing to redirect the site correctly will result in error pages and a drop in rankings. We has carried out this process over and over again and we can assure you we know what we are doing.