I think it's fine to use "retarded" like that if that's your culture and how you learnt it. People will know you're from a different background, maybe very old or foreign, or whatever. But if you're just trying to force an old meaning onto it, people will misunderstand you since you're not using the common "protocol".
I guess the problem is it's hard to pinpoint when precisely the "current" meaning becomes the "old" one. This is doubly difficult for meanings which contradict each other (see "woman").
What happens when a substantial portion of the population refuses to adopt the new meaning? It seems like the language gets forked then. This doesn't seem to do any good at all, increasing polarization and creating obstacles to clear communication.
