If a female HRH marries a male HI&RH their children would be HI&RH. Once again, the question of female Heads of non-reigning Houses is extremely hypothetical as female Heads are few and far between. Grand Duchess Maria Vladimirovna is the only notable example.
(Crown) Princess Margarita of Romania will in due course become the Head of the Royal House of Romania. She is married but has no children and will be succeeded by her sister Helen's son Nicholas. Nicholas Medforth-Mills, since renamed Nicolae de Romania, will become HRH Prince of Romania on his 25th birthday (next year) or on the death of King Michael, whichever comes first.
A couple of more general examples of "marrying up":
• The (now) HSH Hereditary Princess Caroline of Monaco became HRH The Princess of Hanover on her marriage to Prince Ernst August. If she succeeds Prince Albert as Sovereign Princess then Monaco will have a Royal House for the life of her reign. What style she would give the next Hereditary Prince is anyone's guess but it would probably be HSH Prince Andrea and Monaco would revert to a Princely House on his accession.
• When Grand Duchess Charlotte of Luxembourg married HRH Prince Felix of Bourbon-Parma the Grand Ducal House was raised to royal rank in that all of their children were born Royal Highnesses rather than Grand Ducal Highnesses (the Hereditary Grand Duke would have been a Royal Highness in any case).
and a dynastic "what if?" that never eventuated...
• HRH Princess Astrid of Belgium married HI&RH Archduke Lorenz of Austria-Este, now Head of the Ducal House of Modena. With the revamped Belgian succession law she jumped a place to be behind her brother Philippe and before her younger brother Laurent. If Philippe had remained unmarried (or childless, or left no surviving issue) she would likely have become the first Belgian Sovereign Queen. Her eldest son Amedeo, HI&RH Archduke of Austria-Este, Prince of Hungary and Bohemia and HRH Prince of Belgium, would in due course have inherited the Belgian crown. In effect (though not in name) a scion of the Habsburg Dynasty would have gained a Kingdom more than a century after the family had lost an Empire, and the Habsburgs would again rule over their ancestral lands that once formed part of the Duchy of Burgundy and later the Austrian Netherlands.
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