It now looks like I may need to partially retract the retraction. In addition, I can finally confirm that my Queen Anne dinner plate has been re-shaped.
Instead of a broad generalized statement regarding all dinner plates, the most likely answer is more nuanced. Some dinner plates may have been shaped before assay, others were shaped immediately after returning from assay, and some plates were re-shaped at a much later date.
I sent an inquiry on this subject to the London Assay Office who kindly referred me to "The Price Guide to Antique Silver" by Peter Waldron and recommended that it had "several good tips on how to identify fakes including altered plates".
Indeed this reference has a ton of great information including sections on how to identify altered and fake items ranging from mugs, tankards, table silver, porringers, coffee/tea ware and more. A lot of the identification effort focuses on how items were made and warns of marks and seems in locations they ought not be.
An excerpt from Waldron regarding altered dinner plates, and the re-shaping of plain circular dinner plates with Queen Anne/George I marks on pg. 124:
rather than deliberate fakes, there are many plates which have been altered to accommodate the latest fashion, and in these cases one can usually tell by the hallmarks, as they are either stretched, in an unusual place, or partially worn away. For example, the early 18th century plates which were of plain circular design bearing Queen Anne or George I marks, were often re-shaped in the 1750's or later, and applied with gadroon borders. Silver soup plates have never been particularly fashionable, and as dinner plates are more popular, being more useful, occasionally one will find that a soup plate has been converted into a dinner plate by hammering out or cutting out the depth of the bowl. This should however be readily discernible as there will be either crease marks or an unnatural seam at the booge. [the booge being the transition between the central well and the rim of a plate]
Waldron states on pg. 126 that the 5-lobed or cinque-foil shaped dinner plates were first introduced c1740. From this alone, it can be concluded that my plate dating to 1704/1705 has been re-shaped: