Indeed, some units have been granted an exception under maximum annual running days (I think its 5 weeks a year but dont quote me on that) to preserve them but all other units need to be wheelchair accessible along with other disability reqs like a different colour around the doors to raise visibility, minimum door widths, minimum seat spacings, wheelchair/pram spaces, grab poles and seat holds have to be in a certain location and size and painted yellow.
Heres a copy of the regs, theyve just legislated to create seperate versions for light rail and heavy rail vehicles with the split occuring 6 April (previously heavy rail regs applied to light rail with a long list of exceptions and exemptions), however since they were based on heavy rail regs theres at the moment no difference between them (for example regs on catering cars and toilets while no UK light rail vehicle has either or is likely to ever have either). Having seperated them however allows them to change in future allowing each to evolve to its own needs.
http://www.opsi.gov.uk/si/si2010/pdf...0100432_en.pdf
A handful of heritage units are being converted to the new safety regs for general use (touristy routes) and another handful could be used on special occasions with the new fleet forming the bulk of services.