If you had a defence at the time when you did it (i.e. airsoft skirmishing) then you're good. If not, you can't undo the heinous crime, although you can hide the evidence. That's perverting the course of justice though, a far more serious offence.
If that sound like a silly argument, well, it's a damn silly bit of legislation, and not something that you should lose any sleep over.
It's legal now. There's no offence of owning a RIF. Possession in public is an offence whether it's a RIF or an IF, with the same reasonable excuse - being on the way to or from an airsoft skirmish.