We cannnot ignore the 1923 Constitution as a historical document because as you say, it made King Michael Head of State as King of Romania in 1927. As a consequence, his current position under the current arrangements is former head of state. This puts him on a par with all other former heads of state under all former constitutions (1866 through 1991), from Carol I to Basescu. I don't see how this can be undermined or changed because it's historical fact.
Today, in real terms, the 1923 constitution has been binned, rejected, consigned to history. It no longer has any legal status in Romania, having been replaced by, most recently, the 1991 version (with amendments). In France, the constitution of the 5th republic replaced that of the 4th. It didn't erase the constitution from history but it did erase it from the present. Just because one acknowledges that a document has passed into history, it doesn't mean that one disrespects it as toilet paper.
King Michael has never presumed to have the right to change any constitution. As his decisions to create his own house, severing links with the Hohenzollerns, and to designate his daughter future head of that house, have no constitutional impact whatsoever, Romania having a republican constitution, he cannot be accused of having changed (or tried to change) any constitutional rules one-sidedly.
He has, however, requested that the succession rules of his house be adopted if Romania should ever decide to restore a monarchy. Adopting the position that the rules are "frozen" at the moment of the abolition of a monarchy creates a major problem. It obliges the heir to defend or maintain the civic rules of a former time - and the social norms that underpinned them - which have since been rejected by the society that made them.
Among many former houses across Europe, from France, through Italy, Saxony, Romania and Russia, we see dynastic disputes between "frozen rules" and "evolved rules" factions, accusing one another of being either outmoded and divisive or whimsical and divisive and suggesting that either by moving with the times or by not moving with the times, the faction in question is damaging the overall cause. On that score, both sides are probably right.
