I did it on day one. No issues at all. The LED festoon bulbs are a few mm longer than the standard bulbs, so it just means its a slightly tighter fit, but the brass terminals they snap into bend anyway.
I think this is the problem Craig might have had, that the LED versions had made the "gap" slightly wider between contacts, so when he put his standard bulbs back in, they didn't fit snug and rattled. A quick bit of pressure on one of the electrical spring tabs, would have pushed them back together to grip the standard bulbs tightly.
Also, very importantly as Stuart said, LED bulbs are polarised, and only work when installed in a particular direction. So if at first it doesn't illuminate, just spin it 180 degrees and it should then work.
Really easy....shame on you Craig