Well, we never did come up with an official term for combing two dresses into one garment (Whatsitsname), but I'm glad to say, it's done! I unpicked, re-positioned and re-stitched to produce a dress that not only fits me better, but also looks more up-to-date and stylish.
I was also very lucky with the lengths of the two dresses - they were really very close. For a few brief moments of joy I had thought that they were exactly the same length, but actually there was about half a centimetre's difference. Luckily the length belonged to the plain dress, so it was a simple case of lowering the floral panel a little, rather than the whole dress. Phew!
I'm pleased with the finished result, and even had it done in time to wear to a summer wedding. Project completed and on time - result!