Fellowsfilm have owned both .co.uk and .com for some time, with the latter redirecting to the former. The change is simply to reverse that behaviour, so that .com is the primary domain. Unfortunately the reconfiguration has broken some things, and so links with .co.uk in this are currently returning 404. It's being fixed as we speak.
The change is simply because the .com domain should appeal better to a world-wide audience. We're trying to dispell the idea that we're a purely UK OMSI forum, but simply an English-speaking simulation forum that's home to the UKDT and a lot of UK members.