Just like on your average soap opera - where a major character has a car accident, and is swathed in mummy-like bandages from head to foot, and when they come out of hospital it is an entirely different actor . . .
Obviously Pr. Catherine has LTB and taken the sprogs with her. The palace has come to a financial arrangement with her - because she is so popular they will keep the character, but get a different gorgeous, leggy, uber-talented, sporty, charming, musically adept sloane to take over the role.
She will look nothing like HRH but that doesn't matter as it will be attributed to stress and abdominal surgery - and anyway, as the producer of "Acorn Antiques" said when Mrs Overall walked onto the set ostensibly to deliver tea, with her arms outstretched but no tray, "We professionals pick these things up - the public won't notice.
YOU READ IT HERE FIRST, FOLKS!
Edited because "sprogs" come out as "sprigs"