I noticed that too. It makes sense as the Imperial State Crown is heavy and I thought that the stage management was well thought out and non alarming. It was very much she 93 and this is more comfortable than OMG she's not wearing the crown what's happening. Very well executed...and I have a soft spot for the Diadem and wish we saw it more.
It makes very good sense since she now sometimes hunches over as she walks. It would be horrid if the ISC fell off while she was walking. Nicely done substitution with the diadem. She has wisely opted for a smaller diamond necklace too.
That said the Imperial State Crown is truly something to behold.