Does a convalidation validate the original vows or is it basically a new wedding, and afterwards the couple is married from that point on but not before?
Convalidation is not retroactive. The marriage is considered valid from the point of the vows exchanged at the validation.
A retroactive version is called a radical sanation. In this process there is no new vows, the bishop accepts the original vows. For more information about* radical sanation* see: Radical Sanation of a Merely Civil Marriage
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