The Luxembourg grand-ducal family already owns a nice amount of tiaras - of course not as many as their British cousins, but definitely more than their Belgian or Danish ones, for instance.

Their problem rather lies, IMHO, in the (relative) lack of necklaces. Most of the Luxembourg tiaras do not have a matching necklace, so most of the time the Luxembourg Grand-duchesses and Princesses either wear a tiara without necklace, or make a pairing which does not look very satisfying (e.g. a neoclassical tiara with an Art Deco necklace). It's still better than nothing, but not perfect anyway.

So I think there is no need to turn necklaces into tiaras, but rather to purchase better-looking necklaces to match the available tiaras, instead of tinkering with bracelets...