The law is that "[modifying] an imitation firearm so that it becomes a realistic imitation firearm" is an offence, see
http://www.legislation.gov.uk/ukpga/2006/38/section/36
There's no distinction between permanent and temporary. The offence is committed each and every time that you do it.
Doing so for airsoft use is a defence. If you've got a skirmish booked at a site, I'd argue that you're good to go since the defence is about future intentions, not past behaviour. However, it would be better if you've played, and are booked to play again. And best if you've played at least three times over at least two months, and got UKARA or equivalent.
Other opinions are available, this is not legal advice, and I'd stress that the defence is on you to adduce, so "intending to book" won't cut it.
If you already have RIFs, those are good. You don't have to do a thing to them, as ownership isn't an offence.
However, being in possession of any imitation firearm, realistic or otherwise, in a public place is an offence, see
http://www.legislation.gov.uk/ukpga/1968/27/section/19
So whenever you're on your way to or from an airsoft skirmish, have details of where you're going, or where you've been to hand. This applies to everyone, regardless of whether we have UKARA or not - what matters is the reason for being in possession at the time when it comes to the attention of the State.