Cobham shares in London were down 2 percent at 228.15 pence by 0820 GMT.
The British company said there had been no technical issues with the conversion process and that it would suffer no material financial impact as a result of the change.
The project has been dogged by controversy since the MoD agreed the deal in 2008.
In 2010 the UK Parliament’s Public Accounts Committee concluded in a report there were “significant shortcomings” in the way the ministry had contracted to lease the tankers.
However, the government pushed ahead with the deal because the RAF urgently needs replacements for its ageing Lockheed Martin TriStars and Vickers VC10s.
The MoD was criticised two years ago after it was revealed that Voyagers lacked the protective gear necessary to operate in war zones.
Earlier this year AirTanker planes encountered leakage problems during in-flight test refuelling of British Tornado jets and they face compensation claims from the UK defence ministry if the problems result in delays or extra costs.
Nine of the planes are due to enter service in 2014.
The AirTanker consortium is made up of EADS, France’s Thales , in-flight refuelling pioneer Cobham, Rolls-Royce and Babcock International.