PICAXE's cannot have firmware upgraded, so you would have to wait and buy a 14M which has firmware support for READTEMP12 added, or switch to another PICAXE which already does.
The deciding question is, will the 14M firmware support READTEMP12 in the future ?
A stop-gap or semi-solution would be to use your 14M to control a PICAXE-08M which did the READTEMP12 and passed the data back to the 14M.